I would change my password and set up a user account just for them that was locked down secure.

It would depend on where the virus came from. If they were downloading illegally and got a virus, or got it from a porn site, I would be furious. Since most viruses come through emails, it would seem that someone needs to educate them about not opening emails from people you don't know, and not opening attachments on someone else's computer. If it was just a true accident, one of those drive-by sites or something, that would be different.

It depends on what drive/files or if it's the system that's been infected. Prevention is always better than cure, they say. That's why I always make it sure that I have my anti-virus on and updated daily so I could prevent viruses from infecting my computer. You don't have to worry, you can scan your computer and then clean the viruses out of it. Or you may deep freeze (bullet proof) your drives so no one can touch your files. When you restart your pc, your files are intact and safe.

I would notopen any of their attachments, and I would warn them not to accept attachements form people they did not know or emails on topics that made no sense. I would also make my computer off limits to them if they did it while downloading something here on my computer.
I would distance them from my computer, no matter how it happened.

Use any of the antiviruses for virus protection

eTrust EZ Antivirus
McAfee Antivirus
avast! Antivirus
Kaspersky AntiVirus

Step 1.
As soon as you suspect that your computer has a virus, remove your computer from any networks it might be on, as well as from the Internet, so that you don't inadvertently spread the bug to others. Unplug your network cable if you have to.

Step 2.
If you have virus-scanning (anti-virus) software installed, run it.

Step 3
If you don't have anti-virus software, you'll need to obtain some. If you can't get it from a network administrator or download it from an uninfected computer, you can mail-order it from a retailer.

Step 4
Start your computer (still not connected to a network) and follow the instructions that came with the anti-virus software.

Step 5
Keep running the virus-scanning software until your computer comes up clean.

Step 6
Reconnect your computer to the Internet and check with the anti-virus software's publisher to make sure you have the latest updates. If not, download them now.

Step 7
After updating the anti-virus software, run it again until your computer comes up clean.