I built my system almost 2 years ago and I have not encountered any problemslike this until just last night and it's been happening since then. It will suddenly lock up 2-20 minutes after starting up. I've been monitering the CPU/memory usage and the core temperatures when this occurs but nothing seems to really stand out other than the fact that my hdd light on my case will stop blinking, but this seems to be a result of the freezes.
I considered doing a system restore but I chose not to in fear of crashing halfway through the process.
The only probable cause for this problem that I can think of would be the system updates I completed shortly before I first noticed this problem. Here they are:
•definition update for windows defender - kb915597 (definition 1.61.1497.0)
•Internet explorer 8 for windows vista for x64-based systems
•Microsoft .net framework 3.5 service pack 1 and .net framework 3.5 family update (kb951847)x64
•ralink tech Corp. -network-linksys-wireless-g pci adapter
•update to .net framework 3.5 service pack 1 for the .net framework assistant 1.0x64 (kb963707)
•the 2007 Microsoft office suite service pack 2
•windows live add-in 1.4
•nvidia - display - nvidia geforce 9800 gtx / 9800 gtx +
and my pc specs are:
•CPU:intel core 2 duo e8400 @3.0ghz
•memory:4gb (2x2gb) ocz reaper oc'd to 4-4-4-15 timings
•hdd:2 seperate hdds for total of ~800gb
•gpu: evga geforce 9800gtx
•sound card: creative soundblaster x-fi xtreme gamer
•os: vista home premium
•mobo: gigabyte ep35-ds3l
System lockups/freezes are often caused by RAM problems. I would start off by testing the RAM using Memtest86+. If you get any errors you either have a dead stick of RAM or need to adjust the speed/timings/voltage.
The Creative sound card isn't going to do you any favors, either. They're a common cause of system instability also, but I doubt it's causing your problems unless you recently added it.
If the Memtest86+ runs for at least 4 passes with no errors, I would download the latest GPU drivers directly from Nvidia instead of using the ones Microsoft found for you.
What brand are your hard drives? If they're Seagate 7200.11 drives that could also be the source of your problems. The 7200.11 drives have well known firmware/reliability issues.