Random Freezing Issue

My machine has recently begun randomly freezing on me, when I am on it and while I have been away. I haven't made any major changes to it recently, software or hardware-wise; just a new cpu fan since my old one started making weird whiny noises and I decided to open the case and it was stuttering.

I have not found any consistency in the freezing. I'm usually running Firefox 2.0.0.14 and RealPlayer 11 when I'm on the PC. If music was playing, it stutters and falls to silence and thats where I'm stuck. It will be up for days w/o a problem and then freeze, when I reset to reboot it'll be fine for a few hours then freeze again. Even this is not consistent. This is my last hope before trying a reformat.

The specs:
AMD athlon xp 3000+
Chaintech 7njl6 motherboard
1.5GB ram (1gb, 512mb - Ultra pc3200 DDR)
Ultra X-Connect 500W PSU
nVidia GeForce MX4000 agp
Hitachi 160GB master hdd
Seagate 40GB slave hdd (I think its a Seagate, it was salvaged from my previous Compaq PC)
Win XP Pro SP2 (worried installing SP3 will force me to reformat because of this issue)

I don't troll strange places on the web, but still run Spybot - S&D monthly at least, and Virus scans weekly.