I got the same problem

AMD Sempron 1.6ghz 64-bit
Abit k8-ux motherboard (onboard sound, network, raid and firewire)
1gb + 256mb pc3200 ram
80gb maxtor sata HD
Geforce fx 5200 128MB agp 8 card
Thermal Take silent 480w power supply

Random lock ups. But most evident while watching video files. I have a 60 minute divx file which i have been througha round 12 restarts and have still not finished watching, the most recent attempts it locked up 3 times within 2 minutes so gave up.

I only play counter strike so only 1 game to test, but it locks that up too.

Other problems include random explorer crashing, with either no reason, or due to skype plugin (now disabled)

Tried various nvidia drivers, also tried a set if individial codexs and wm player classic (as suggested by another site)

From reading across the website and my own experience, this seems heavily video related. I read that even with browsing websites sometimes ffdshow is sometimes used to render previews and videos. Could be a bug only surfacing with particular hardware or could be bad drivers. who knows.

The last thing i may try is to change to a new graphic card which supports vista. Failing that its back to xp.