Hi Kafhe, welcome to the board.
Sorry to hear you are having random freezing problem. This could be anything from software to hardware problem. If update BIOS and what smarteyeball's suggestions wouldn't help. So here are a few thing I would try start from something simple.
First, check to see if all components and connections are seat and secure especially the rams and the SATA connections. Shake and wiggle, disconnect and reconnect the SATA connection at the HDD to make sure it is secure. Replace them with aftermarket ones if you haven't done so. This is a commonly easily overlook problem for random freezing.
If not helping, run a couple of runs of mentest and chkdsk to see what happen.
Next, you can run the unit with minimal components. Such that you might use one stick of ram, no board gpu and sound card. Then reinstall them one by one.
If all the above check passed and problem persist, backup you data and clean reinstall Vista.
Good luck and let us know how things go.
Bruce