hi,
First, I would update all of your major drivers. Video, sound, network,
chipset, and SATA controller (the last 2 are included in the same
package in some cases. Check your motherboard manufacturer's website for
more info). Once those are up to date, play around with th system for a
bit and see if it still freezes up on you.
You were definitely on the right track when you mentioned doing
those RAM tests. For that, go download Memtest86+. If the RAM checks
out, make sure your temps are all ok (lots of utilities around for this
purpose - I happen to like HWMonitor from CPUID). If you aren't sure if
the temps you're getting are within thermal limits for the hardware,
post them here and we can find out for you.
If those are ok, I would look at doing some diagnostics on the hard
drive. The Ultimate Boot CD (www.ultimatebootcd.com) has lots of good
HDD diagnostic tools.