BOAC,
The little bugger may be coming back from the system restore files, I have had a similar thing happen in the past. Try turning System Restore off (all the restore files will be lost). Then delete the infected directory.
Run msconfig if you can, and have a look in the services and startup tags and see if anything odd is present, untick anything that looks dtrange (you can alwats retick later)
Odd start files that contain random alpha characters are a good bet.