(on a computer with 400 infected viruses they found 1 infected file).
how do you know you have 400 infected files?

tools that can be run from USB

ClamWin Portable. http://portableapps.com/apps/security/clamwin_portable
superantispyware portable. http://www.superantispyware.com/portablescanner.html
Norman Malware cleaner. http://www.norman.com/us/home_and_small_office/trials_downloads/malware_cleaner

and there are probably some i forgot…

if you need removal help, follow guide, attach logs and help will arrive.
http://forum.avast.com/index.php?topic=53253.0