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Backing up installed software often happens with system backups, but it's not enough. You need additional steps to properly backup installed software.
How can I backup software. I always download free licence software from web. I installed the software then I backup it using program selection. I notice that when I restore the software they ask me to register the software. During Installation I register the software by activating it in the internet, but during restore it ask me again. How can I avoid reregistering the software?
I'm not exactly certain what you mean by backing up using "program selection", so I'll cover the more general case of backing up your installed software.
There are several ways to backup the software that you purchase or download for your machine. The problem is that most people seem to neglect what is perhaps the most important backup of all.
If you do nothing else, you must do this:
Save the media: CDs, DVDs or other installation media for any physical product that your receive.
Save the product keys: by this I mean the activation or product keys that are printed on the box or otherwise included with physical product.
Save the download: immediately after downloading a product, and ideally before you even install it, save that downloaded file somewhere.
Save the emailed activation codes: for software that requires online registration and activation save the activation keys, codes or registration information that's provided.
By "save" I mean back up, copy to a safe location, burn to CD or DVD ... whatever works for you such that next month, next year or even several years from now you can retrieve that information should you need to reinstall and re-activate that application.
And yes, you need both: the original download or installation media and the accompanying activation key if the software requires one. Without the first you'll not be able to reinstall the application, and without the second you won't be able to activate it again.
Why all this effort? Simple: one of the most basic "recover from anything" problem solving techniques is: reformat and reinstall everything. In fact, it may often be the only solution if, for example, your hard drive fails. Unfortunately, if you don't have the applications to reinstall, you'll have lost them. If you don't have the activation keys to get them working again you might as well have lost them.
The other common scenario for wanting both the media and the installation keys is simply deciding to move the application to another machine. Once again this requires a reinstall and reactivation.
If you're performing what I'll call "traditional" backups - periodic system, image or other backups then you may have an additional layer of protection. I say "may" because not all backup configurations actually backup installed applications; if your backup is simply of your data files then it's very unlikely that your system and installed applications are being backed up. On the other hand, if you're doing an image backup of your entire system then all of your installed software almost certainly is being backed up.
If your image backup is taken after you've installed and activated your software, then restoring that backup to the same machine should simply work - there should not be a need to reactivate the software again. Of course if the image backup was taken before the software was activated, then that's the state you'd be restoring to: not yet activated.
Things get interesting if you restore your backup to a different machine. For example, if you restore it to an additional drive on a running system - say the D: drive - this does not automatically "install" the applications on that system. While the files that were installed on your original hard drive are there, the additional information that is installed to the system registry and elsewhere is not present in a way that the running system will recognize it. In short: you can't move installed applications from machine to machine this way. You need to reinstall from your original media or downloads, and reactivate.
If you've backed up your system drive, and you now restore that to the system drive of another machine (and that machine is sufficiently similar to the original to keep Windows happy), then things get a little iffy. In general, I would expect installed applications to simply resume working without requiring activation.
But here's the catch: there's no standard on what "activation" really means, or when it's triggered. That's all up to each application. It's unclear what changes to a system might cause a particular application to want you to activate it again. It's possible that some applications might take note of differences between the old and new system and decided that enough had changed that activation should be required again.
While that's certainly somewhat of an inconvenience, it's not actually something I'd worry about too much. Though it is another clear reason for making sure that you've saved your product keys in a safe place in case you need them for situations such as this.