Okay, I think I've finally found out what's causing my freeze-up problems.
Bad DIMMs on my mobo.

(In layman's terms, the RAM slots are dying.)

I ran the Sysoft Sandra test suite, and it reported that I had 128MB SDRAM in each of my 3 DIMMs.

That ain't right.

I have a 128MB stick in slot 0, a 256MB stick in slot 1 and slot 2 is empty. PC Pitstop's online tests also had trouble seeing what RAM I had, and returned different amounts everytime I ran the test.

I know it ain't bad RAM. As I said in my earlier post, I swapped it all out, and the freeze-ups still happened. So that leaves the DIMMs as number 1 suspect.

Lucky for me, I have a buddy working in a computer shop who's going to give me mate's rates on an upgrade.

(edit: typos)