Summary: Sometimes the best approach to resolving an issue, be it a virus or simply software rot, is reformat and reinstall. We'll review the steps.
In many of your answers you talk about "reformat and reinstalling Windows". What is that? How do I do that, exactly?
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"Reformat and reinstall" is the computer's equivalent of erasing the chalkboard and starting over.
And I do want to emphasize the "erasing" part.
It's conceptually very easy, but also somewhat time consuming.
And if not done with the proper preparation, you could lose absolutely everything on your computer.
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Reformatting and reinstalling is pretty much exactly what it sounds like:
Reformatting is the process of initializing a hard disk to an empty state. This step deletes everything on your hard drive. Everything.
Reinstalling is the process of copying Windows back on to your hard drive and reconfiguring it to your needs. It also encompasses the fact that you'll also need to reinstall all the application software you use, as well as restoring all the data you might want to continue to have available.
If you have an existing computer that you're about to reformat and reinstall, there's a very important first step:
Back Up
I don't care how - be it an image backup of the entire machine (my preference), or a careful backup of all the files and any other data that you want to keep - but you must do this before you begin. The process we're about to embark on will erase everything from your hard drive.
You'll also need all the installation media for everything you plan to install, including your original Windows installation disk and its product key.
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There are two basic approaches:
Reformat or erase the disk, and then run Windows setup
Just run Windows Setup, and let it reformat the disk
I'll spend most of my time on the second option, since it's all that most people need.
The first option gets complicated since you can't reformat or completely erase a hard disk that's in use - i.e. if you've booted from it. If you want to take this two-step approach then my tool of choice to erase a hard disk that's been in use is DBAN. Boot from that, erase the disk, and then boot from your Windows installation media to reinstall Windows from scratch.
As it turns out, the Windows setup program can be used to erase or format the hard disk as well; it's just not obvious where in the setup process you can do so. We'll examine it both for Windows Vista, and Windows XP, since they're quite different.
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Window Vista
Step one is to simply boot from the Windows installation DVD and eventually the Windows Vista Setup program will begin:

You'll then walk through a series of configuration pages:
You'll finally come to this page:

Regardless of whether or not "Upgrade" is available to you, choose Custom.
After a few minutes of examining your system, Windows Vista Setup will then display this page:

This is where you tell Windows Setup on which drive you want Windows installed. Note that the list of drives on your system will likely be different than seen here.
Click on Drive options (advanced):

Note that several additional options are now visible.
First: Make sure to click on the drive or partition you want to erase and on which you want to install windows.
Second: click on Format.

Heed the warning: "If you format this partition, all data stored on it will be permanently deleted."
Press OK, and Windows Setup will format the hard disk partition that you've selected.
After the format is done, you're returned to the drive selection page, where you can click on Next to continue the installation.

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Window XP
Step one is to simply boot from the Windows installation CD and eventually the Windows XP Setup program will begin:

After accepting the EULA you'll be presented with a list of prior Windows installations:

We want to install a fresh copy of Windows XP, so press the Escape key (ESC).
The next screen shows us the available drives and partitions:

In order to force a reformat, we're going to delete the destination partition and recreate it.
Click on the drive that you want to install to, and press "D":

Press Enter to confirm that you want to delete the partition. Since this is a destructive operation (all information on the partition will be erased), Windows XP Setup asks again, just to be sure:

Press "L" to delete the partition.
Now you've got an unpartitioned hard disk. Windows XP Setup now presents you with the list of drives and partitions again:

Select the unpartitioned space, and type "C" to create a partition. You'll then be asked to enter the size of the new partition:

The default value should be the maximum appropriate for that partition, so simply press ENTER to continue.
Setup returns you to the partition selection screen:

This time what was the unpartitioned space is now "Partition1 [New (Raw)]". Select that and press ENTER.
Windows XP Setup notices that your destination partition is not formatted:

The default is to format using NTFS, which is just fine for more situations. Press ENTER to let the format begin.
Once the formatting is complete, Windows Setup then automatically begins copying files to the newly formatted partition and continues with additional options and selections related to how you would like your new installation to be configured.
Related:
I don't have an installation CD for Windows XP - what if I need one? At some point you'll need your Windows installation CD. If you don't have one, you could be out of luck. Be sure to get one and keep it safe.
Don't lose your keys! When you purchase software you're not always buying what you think.
What's the difference between an upgrade and an upgrade? With all the different versions of Windows it's easy to get confused, particularly when the term "upgrade" can actually mean two different things.
Article C3889 - October 14, 2009
If you need your key prior to doing the above, all you have to do is download and run Belarc Advisor, and it gives you all of your licenses and all programs, updates, etc. But if you have more than one OS on a disc (as I do), then you need to run it on both partitions.
Posted by: Charles Tilley at October 21, 2009 12:23 AMhow can i repair if bios corrupt
Posted by: faizul at October 21, 2009 5:07 PMthe instructions are helpful , this is the first time i read some thing interesting on Leo, i am excited
Posted by: Kabir at October 21, 2009 10:03 PMi never formatting laptop before and these articles really helpful. but it seems formatting the XP version is quite complicated..
Posted by: yasmin at October 23, 2009 7:57 PMDear Leo,
Posted by: Johnson Chacko at October 23, 2009 9:25 PMI am having a PC Intel 3.00 GHz, 1 MB RAM, 40 GB + 120 GB harddisks, 512 MB Nivida AGP Card. Whenevr I install Windows XP SP2 (Genuine) I do have any problems. But when I update it automaticlaly to Windows XP SP3 and Internet explorer 8, my PC goes very slow. What could be wrong? Should I have to keep XP SP2 as it is or should I have to update it every often through automatic update option including with critical security updates?
Dear Leo,
I have been trying to install Windows Vista on my Intel 3.00 GHz, 1 MB RAM PC, but I was getting an error while installing it. Is my PC compatible for the said installation. Why that does not work on Pentium IV PC while other OS works?
24-Oct-2009
Posted by: Johnson Chacko at October 23, 2009 9:29 PM
Yes, great info, but how, how, how do you reformat Windows 95????
28-Oct-2009
Posted by: Lee Nelson Guptill at October 27, 2009 10:07 AM
I always find great help everytime I open this site.
Posted by: David Wanyiri at October 28, 2009 12:31 AMYes this is the right way that I have been using to Format and Install Windows
Thanks for the info Leo.
One question (or maybe more); I have Vista and want to install 7, but want to start with a clean hard drive. I know there are issues with this and I've gone to the various websites, but my main question is, how can I reformat my hard drive into 'x' partitions so I can keep the OS on it's own and have everything else on another? I have a 250GB (C:) and a 500GB (D:) hard drive (in addition to a 2TB external) and would like to:
Wipe both internal drives clean
Partition the existing C: as 50GB (for the OS) and 200GB for programs.
So in the end I’d have:
C: - 50GB
D: - 200GB
E – 500GB
I used to use a floppy disk and Fdisk my way through, but I don't have a floppy drive and truthfully don't remember how I did it when I went from XP to Vista (I did reformat).
Is it possible to reformat so I'm working with a clean hard drive? I'd hate to load 7 onto a drive that has had 2+ years to collect junk! Is it as simple as doing a clean (custom) install? Will that let me reformat before I load the OS?
Thanks in advance for any help you can provide!
Joe
30-Oct-2009
Hi, Leo.
Any idea why I cannot select F8 to accept the End User License Agreement in step two above for Windows XP? My computer allows the page scroll buttons and the ESC button to work, but not F8 for some reason. Thanks!
Posted by: Dave at November 12, 2009 3:48 PM