Since Windows Server 2003, Windows has had a feature called shadow copies. Basically, this allows you to get back to previous versions of existing or deleted files. The feature was later enhanced in Vista to make the "system restore" feature. Namely, the system takes snapshots of important system files your computer needs to boot by creating "restore points". At any point in the future you can revert to such a restore point. This can be because your system stop rebooting or you just noticed that game demo included SecuROM and is still installed even if the game demo no longer is.
The feature works has advertised however, there was no way in Vista to know how much disk space this feature was using or could use. Windows 7 includes new configuration options that control these settings.