Have had similar problems to ASI, have been discussing it with him for the last week or so, but thought I'd run mine past you as well, just for fun as the symptoms are just slightly different. The problems have been getting progressively worse.
If I cold boot, my mobo will boot up, the fans (CPU and PSU) will spin, but no hard disk activity. I've now learned to leave it like that for about 10 mins, then hard reboot. This time, the disks will spin up. I'll go through the GFX card BIOS, Mobo BIOS, hard disks will be recognised, and the first windows progress bar will start. Generally, it will either hang or reboot at this screen if I've not let it warm up long enough, or may hang / reboot at any point during the boot sequence, but I know if it gets past the progress bar, its almost warm enough!
Once I'm in, all appears fine. However I'm now more frequently getting random hangs or reboots, especially when playing media or burning / ripping CD's. Even looking at one of my lowly forums seems to cause it to hang / reboot occasionally now though, and the reboot is accompanied by feedback through the speakers.... which is slightly worrying! Also, if I've left an application open and not used it for a while, the app sometimes crashes.
If I get any Microsoft crash messages, they tend to refer to drivers, however I think thats a red herring.
I've recently replaced the HDD's and PSU, however this has made no difference. RAM seems to be OK, which leaves me with Mobo, GFX card, and processor.
I'm running XP Pro SP2, AMD Athalon XP2000+, 768mb RAM, NVidia GeForce FX5200 256mb RAM.
Strikes me that a fair number of people I know seem to have had similar problems at the same time! Very odd and coincidental...