That's true for many infections

Thanks for posting
For Windows XP and ME, when a PC is infected, many times the files is also stored in the backup of System Restore. By default, Windows prevents System Restore from being modified by outside programs, like your AV program.
See here to resolve the problem

Cannot repair, quarantine, or delete a virus found in the _RESTORE or System volume information folder
http://service1.symantec.com/SUPPORT/nav.n...000092513515106