I've just checked in AVG forums and (apart from a small glitch with FAT32 file systems) AVG Antivirus Series 8 complies and works fine with Windows 2000.
see here: AVG Free Forum :: AVG 8.0 Free Edition - Installation Issues :: AVG 8.0 FREE - Supported Platforms (read the whole article for details about the glitch)
(If series 8 is not suitable because of the glitch and you don't want to change file systems series 7.5 free version can be used instead.)
See here: AVG Free Forum :: AVG 7.5 Installation Issues :: AVG 7.5 FREE - Supported Platforms
Version 7.5 downloads available from site listed below:
AVG Antivirus 7.5 (Free) - Downloads - freeware, shareware, software trials, evaluation - Atomic v2.6 - Maximum Power Computing - www.atomicmpc.com.au
The AVG free product is a very good antivirus application.
That file you referred to could well be the one that's causing all your problems
(Some viruses do disguise themselves with legitimate software names.)
(especially if you are unaware of installing it or are unable to find it's installation folder on your computer in the C:\Program Files section.)

I'd say give AVG a go and let it find whatever it can and if possible fix the problem for you.

I'll just add that in this day and age you'd have to be mad not to run some sort of antivirus and if possible a firewall as well.
There are plenty of reputable products available and the AVG paid version (which includes a firewall and a few little extras that the free version does'nt have)
is particularly good according to my computer technician.