The computer you're using now, does it have a CD burner? I haven't tried it, but I heard that Ultimate Boot CD has an antivirus package onboard.
http://www.ultimatebootcd.com/ Ah, yep, scrolling down a ways, I see the antivirus definitions are badly out-of-date, but you've got nothing to lose by trying.
Another option would be to do a parallel installation of Windows into a different directory than C:\WINDOWS (like maybe C:\WIN2),
patch it to SP2 using the offline installer before letting it get any sort of network hookup, then hooking up the network and getting a free 30-day
trial of Kaspersky Antivirus Personal 6 and scanning the other Windows installation for viruses. And salvaging your data in case that didn't help.