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First off is the matter of determining which virus(es) are infecting the computer. Several of the major antivirus software companies offer internet-based scanning - that's less effective than an installed scan, but it will give you an idea of what you're dealing with. I would try the web-based scan from Panda Antivirus, as I've had good results with that one recently.

Note: you have to go to that website from the infected computer. If the infected computer doesn't boot, you are approaching the "pay someone to fix it" realm.

Once you have an idea of what virus(es)/worms you're dealing with, consult the Symantec Security Response website for removal instructions and/or tools. Ease of cleanup varies, depending on what you've got and how long you've had it; sometimes you have to just "nuke and pave" (meaning format the hard drive and reinstall EVERYTHING). I'll tell you up front, last year I made some good freelance money cleaning up people's virus-infected PCs.

Now, on to the issue of ongoing protection... The fact is, the company needs to pay for antivirus software. There is no free antivirus software available for businesses. My personal recommendation would be either Symantec (also known as Norton) or Panda. The issue of antivirus software is a matter of pay now or pay a lot more later.

Finally, you mentioned that this came about because a soon-to-be-unemployed French teacher was trying to download a program over KaZaA. I have a few thoughts to offer on that matter:

This is a business computer. KaZaA software has no place on a business computer. Not only is it spyware, but it is a tool whose entire purpose is to circumvent copyright and licensing laws. What someone does on their home PC is their business (not that I support piracy in any instance), but if someone installs copied/pirated software on a business PC, the business is liable. The Business Software Alliance is the organization that polices this - check out the penalites. It's really not worth it... if you installed MS Office from one single-license CD set on 2 computers, that's up to $4 million in potential fines, plus potential jailtime.
I am aware of the cost of purchasing software and licenses, including antivirus software. As a matter of fact, just last month I was pricing antivirus software for my network at work (we ended up spending about $875 for 25 licenses of Symantec Antivirus Small Business Edition, by the way... and we did NOT get hit by Blaster or Sobig or Mimail last month). This is part of the cost of choosing to run Windows - part of the trade-off for going with what the masses use. If the computer contains important data, it's necessary to protect it.
The business owner needs to set in place a policy addressing the issue of software - what is installed on the PCs, by whom, etc. He should spell out specifically whether or not people are allowed to download software from the internet onto the business PCs... and I'd suggest he make it clearly not allowed. All software installation should be done by the individual(s) responsible for maintaining the systems and/or network, and that person should be responsible for keeping track of the licensing. Sample policies can be found at TechRepublic. You have to join to search for things, I think, but there's a bunch of good info there anyhow and it's free to join.