In many cases, a virus will load itself into your system restore files. It is a technique used by hackers to keep re-infecting your computer using its own back up system. So anytime you are infected with a virus, make sure that you disable the system restore feature and then run your virus scan.
Disable System Restore in Windows XP:
Right-click "My Computer" and select "Properties". Click the "System Restore" tab and check the box "Turn off System Restore" then click OK.
Disable System Restore in Windows ME:
Right-click "My Computer" and choose "Properties". Select the "Performance" tab then click the "File System" button and go to the "Troubleshooting" tab. Put a check mark next to "Disable System Restore" then click OK.
You'll need to restart your computer.
Run your virus scan. Once it is complete, follow the instructions above in reverse to Enable System Restore.