Just to add something to this discussion...
I have a very similar problem, and I am using an Enermax Whisper 350W power supply. I've been looking for a solution to this for a little while now, and so far the only things I've heard were about the power supply and the memory.
I originally thought it was my Audigy card, so I took it out, and I've only had one crash since. However, the freezing seems to occur when I am using Internet Explorer and moving the mouse, which leads me to believe it's something to do with the PCI bus. I am overclocked, 1.6A @ 2.13, default voltage, standard heatsink/fan, and it runs consistently at around 35 - 43 degrees celsius, so I know it shouldn't be because of overheating.
The problem is not a Windows XP problem, either, since I was using Gentoo Linux a little while back, and it would do the same thing. It must be because of sunspots or something. Who knows?
Specs, in case anyone cares...
P4 1.6A @ 2.13 (1.5V, Intel Heatsink + fan)
MSI 645 Ultra
Nanya 2x256 PC2400
Leadtek GF3Ti200 64MB
Netgear 10/100 NIC
Linksys Phoneline 10mb NIC
SB Audigy on my desk
2 IBM 120GXPs, 60 GB and 40 GB
Pioneer 16x DVD
Generic 16x CDRW
etc, etc...