To remove a particularly pesky virus, you night have to disable System Restore.

Or maybe you have a third party program to handle system restores like Norton GoBack or some such app and you don't want to duplicate the same service and/or gain a little of your system resources back.

Personally, I will avoid all Norton/Symantic products from now on. They used to be great, but are now so bloated and intrusive and have a god chance of screwing up your computer. They seem to have forgotten that the purpose of a computer is not to run anti-virus and maintenance software only.