Helping people with computers... one answer at a time.
Keeping data on your computer secure is important. Being able to password protect a folder seems an obvious approach. Unfortunately it's not that simple.
Can I put a password on a folder so that only I can see its contents?
Yes and no.
You can do something similar to password protecting it using Windows security features. It depends, though, on using the computer the "right" way. On top of that, I actually don't really recommend it. If you have something that you want to password protect and keep secure, I recommend a slightly different approach.
Windows allows you to place restrictions on who can do what with a folder, or even a file. In Windows Explorer, right click on a folder and Properties, and then click on the Security tab:
Here you can see the properties of a folder on my machine called "books".
Here you can control who has access to that folder. The default way my machine is set up, everyone can examine the contents of that folder. I can remove that and further restrict on an account-by-account basis which users can access that folder, and whether they can modify, read or even see the folder contents.
It's actually very powerful, if a tad complex.
However, it's based on Windows user accounts. Thus if you give your own account full access to the file, as I assume you would, then anyone that can login to the machine as you can immediately access the file. There's no real password on the folder, it's your ability to login to Windows using your login password that controls your access to the file.
And since it's based on Windows user accounts it assumes you're actually using different user accounts for different people. It's very common for that not to be the case.
The approach I prefer, and in fact use myself, is to use the free open-source tool TrueCrypt.
With TrueCrypt, you create a single file on your computer's hard drive that is encrypted. If someone looks at that file all they see is random data - there's no way to know what that file contains.
Once you "mount" that file using TrueCrypt, and supply the correct password or pass-phrase to unlock it, the contents of that file appear as another drive on your system.
For example, I might have a file "c:\Windows\secretstuff.tc". There's nothing you can do with that file without TrueCrypt and the password to the file. Since I know the password, I can mount it using TrueCrypt and suddenly a new drive appears - say "P:". That drive then contains all my protected files. I can change them, update them, delete them - whatever. Once I'm done, I can hide them all again by simply unmounting the TrueCrypt drive.
It's both simple and elegant.
And it's not tied to Windows, user accounts or anything else. In fact, you can copy your encrypted file to another machine entirely and mount it with TrueCrypt. Even using other systems such as Linux.
And while any encryption is vulnerable if you pick a bad password, the actual encryption algorithms used by TrueCrypt are "industrial strength" and nearly impossible to crack with current technologies.