I want to check for viruses on a computer that I suspect may be infected with malware.

Its users are running an antivirus, but there's always the risk that something slips past and the way I see it, once the system is infected the antivirus is useless because the malware can hide itself from the AV.

I think the best way to go (besides clean reinstall of the OS) would be to have an antivirus running at a boot time from a CD or a USB key. That way, the malware is just lying on the disk and cannot do any of its hide-and-seek stuff (provided the AV comes from an uninfected PC and all that).

So, I'm looking for something that:

• Runs at boot time (off USB key or CD-ROM)
• Does not touch or require the local OS
• Discovers malware fairly well (like, Avast, AVG, Norton, whatever -- I think the're all the same anyway)
• Can handle Windows filesystems (FAT 32, NTFS, WinFS ;-) )
• Comes from some sort of trusted source (no Windows Antivirus 2009)

I know that this is no silver bullet (nothing is, really*), but I do have a feeling it's more likely to help than doing the scan* within the infected system.