Windows XP and Windows Vista automatically backup the old system when you install new software or updates. They create restore points for the backups. So if you want, you can return to any of the previous restore points.
If you have used the system for quite a while, you would see that your disk C: is getting full because Windows keeps all those restore points from the day you bought your computer. If your system runs quite stable and you don't need to return to the ancient restore points, you can remove all but the most recent restore point.
Here is how you can do that in Windows XP (Windows Vista is similar).
- Login as Adminstrator.
- From the Start button, select Accessories|System Tools|Disk Cleanup.
- In the Disk Cleanup dialog, click the More Options tab.
- Click the Clean up button in the System Restore section.
- A warning pop-up asks whether you really want to remove all but the most recent restore point. Click the Yes button.