There is a lot of files in software (especialy autocad cracks) that my norton is catching with the pinfi virus.
This is quite sticky and it took 3 days to clean my network of it.
to spot it in your registy go to run, type regedit, and follow down through you registry on this path: HKEY_CURRENT-USER\software\microsoft\windows\current user\explorer opening the explorer file to see if its listed.
If it is then it probably has infected your anti virus software too (sugest latest norton, you probably know where to download it )
Also move all your .exe files out of your shared folder until you clean it, to stop it passing on.