Free Anti-Virus?

I work for one of the world’s most incapable bureaucracies and all my school’s PCs will be zombies before they provide me with an anti-virus program. I’ll also lose my house buying everything myself that they can’t get around to supplying. Does anyone know a trustworthy WinXP anti-virus program that’s free or nearly so? I admit such a product doesn’t make much commercial sense, but hope springs eternal…

Thanks for any help,
Lev