Trouble removing W32.Unruy!gen1 with NAV and NIS

Although NAV 2010 and NIS 2010 will detect, block, and quarantine W32.Unruy!gen1, this threat keeps re-appearing on my Windows XP PC following each reboot  - usually associated with the files: services.exe and smss.exe. 

Of course XP, NIS and (previously) NAV are fully up to date.  (During a Live Chat session, it was suggested that I either read the Norton DIY virus removal guide, or pay for the Spyware & Virus Removal Service.)

 

I'd rather not trash the hard disk and rebuild the PC, but do I have any other option?