And if I remember correctly, if you do not go back far enough for the system restore, the virus could still be incorporated in the restore and you will still have it!
I have been very lucky - staying virus free - but if and when - I intend to delete all my previous restore points after removing the virus and then starting restore over again - knowing I have a virus free restore point!
Just for info from the Symantec site:
System Restore option in Windows Me/XP
Windows Me and Windows XP users should temporarily turn off System Restore. This feature, which is enabled by default, is used by Windows Me/XP to restore files on your computer in case they become damaged. When a computer is infected with a virus, worm, or Trojan, it is possible that the virus, worm, or Trojan could be backed up by System Restore. By default, Windows prevents System Restore from being modified by outside programs. As a result, there is the possibility that you could accidentally restore an infected file, or that on-line scanners would detect the threat in that location. For instructions on how to turn off System Restore, read your Windows documentation or one of the following articles:
How to disable or enable Windows Me System Restore.
How to disable or enable Windows XP System Restore.