Antivirus Software for Windows XP, 98, 98se, 95, ME, NT, 2000, Vista, Windows 7 - still supports Win98. Not free, but not hugely expensive.
If you have a hardware firewall to shelter behind, you could consider going commando - but do consider Ghost (or similar) and very regular backups of any data. If you get hit, just re-ghost. An anti-malware of some kind is essential, as there is no way to avoid total admin rights for any and all users.
What is the spec. of the PC, and what is it you require it to do? I've run Win2K happily on a 1998 vintage Toshiba laptop (233MHz P2, 256MB RAM 4 GB disk) - still occasionally use it as a packet sniffer.
A small Linux distro might be a candidate - experiment with a CD-based "live" version without commitment.
SD