It makes some, although not a lot, of difference which operating system you're using.

It is generally considered good practice to have system restore turned on for normal operation. Especially with WinXP, and to a lesser extent with Win2K, the "last working" version that System Restore has on file occasionally can resolve problems during a reboot that you won't "see" happening. Without the backup registry that System Restore maintains, the OS would have to ignore the error, substitute a generic fix, or ask you what to do.

The ONLY time it's necessary to turn it off is when there is a reason to expect that a backup copy of Registry information may contain a virus or other malware. If the removal of a malware item requires you to delete Registry information that was contaminated by the malware, the next reboot may result in replacement of what you deleted and reinstalling of the malware or some of its programs.

Turning off System Restore should be part of the malware removal procedure, when needed; and most procedures where it's needed will recommend that you make a separate manual backup of your registry before turning off System Restore. The automatic restore process can't access a copy that you make manually, but if you screw up your repair it may be better to be able to go back to the "infected" state to start over than to start from scratch.

When you turn off System Restore, and reboot once, ALL PRIOR COPIES of the registry information maintained by System Restore are deleted.

YOU SHOULD ALWAYS HAVE AT LEAST ONE RECENT REGISTRY BACKUP THAT WORKED.

If you don't let System Restore make backups for you, you need to do it manually - and you probably won't, especially if you need to ask...

John