If you have a virus or Trojans that are replicating after your system seems to have been cleaned ( on Win XP ) ..go to properties of your computer and turn OFF system restore ... run your anti virus program ...reboot and run one more time and leave the restore off.

the little buggers hide in there and will restore after each boot... By turning off the restore windows will delete ALL restore points ...at a later date when you turn on it will rebuild a clean point ....hope this helps