Clam Win is a good antivirus, hwoever the last time I knew it wasn't real-time shielding.
I know that Avast 5.0 was working for a while and you might want to give it a try. Avast is up to version 6.0 but the last time I checked definitions were still being released for version 5.0. The final version for Windows 2000 is 5.0.677.
(File name: setup_av_free.exe, file size 52.5MB):http://www.filehippo.com/download/file/cc261542fb11cd431f04e37c3c2d6bfd2fd0c98e0bcb761aaf17d3c6170c1d3b/