hi
did you think to disable your restore system feature if you got windows Me or XP
if not disable this feature before scanning with avast antivirus because Antivirus cannot delete file located in the restore file so the virus stay on the system
To disable this feature GO to :
Start->Settings ->Control Panel ->System
Select the System Restore tab and select the check box deside "Turn off System Restore on all drives"
this will delete all the restore file
so when you scan virus cannot hide in restore folder
when you get ride of the virus enable again the system restore feature
Hope this can help
sorry for my poor english