Not much to go on ;-), but before you go down the possible virus road, suspect he has done something wrong in the "msconfig" situation.
Slow PC's can be due to a lot of things not just virus. EG: loads of unnecessary obsolete files and also a heavily fragmented disc. I would suggest running some utilities before exploring the virus situation.
First, run a full system file check - then a disc defrag - then a cleanup utility. If the PC won't go all the way normally, try doing them in safe mode.
In XP, safe mode = press and hold F8 while rebooting. System File Check = my pc \ right click local disc [probably C] \ properties \ tools \ error checking \ check now. Tick both boxes and restart PC - it will take some time but let it run and restart the PC normally. Then run the defrag [same location but under "error checking". Download a free cleanup utility, I use CCleaner but there are others probably just as good, try here click here
What you try next will depend on the success or otherwise of the above. Good luck.