Windows Vista has a feature called System Restore that automatically backs up registry and system files whenever you install new software or drivers. This feature is useful when you install evil software that makes your computer run really slow. But don't worry, System Restore won't remove Windows Vista.

Using System Restore in Windows

There are two places that you can use the system restore feature from. From within Windows, you can just type restore into the Start menu search box, and you'll immediately see System Restore at the top of the start menu:

Or you can type rstrui into the search box and hit enter. Your choice.

Update

For best results, you should run System Restore from safe mode. If you receive an error while restoring, then use it from safe mode and it should work fine.

You will immediate see a screen where you can choose to roll back the system to the last restore point. You can select "Recommended restore", and just click next, or you can choose a different restore point.

If you do choose a different restore point, you will see a list of restore points that you can choose from.

You'll notice in the screenshot below that the last thing I did was install Virtual CloneDrive to mount ISO images.

Click one of them, and you will have to confirm and then restart your computer to roll the system back.

If your system won't boot

If you can't even get into windows, you can boot off the installation DVD, and choose the "Repair your computer" option on the lower left hand side. Click next on the next screen.

Now choose System Restore from the System Recovery dialog. It will take a few seconds to come up, and you will see the same screen that you would see in Windows:

Click next, and on the next screen select the drive that your copy of Windows Vista is installed on.

Click Finish, and Vista will roll back to the previous restore point. Really pretty simple stuff.