[plt-scheme] Where can I find an online anti-virus that doesn't install on your PC?

I've got 5 PCs that I'm trying to use to train disadvantaged young
people. The problem is they are riddled with viruses and a firewall
blocks me from updating them. The people in charge of maintaining the
PCs won't fix them or give me the admin password (Win XP) to let me
install a new or updated antivirus the centre is being shut down in a
few months.

If they were working, I could still do a lot with them, so I've been
looking for a good online virus scan - but they all try to download a
little .exe onto your PC first, and the settings on the PCs won't
allow that.

Suggestions? Solutions? Links?