SL siezing up computer - please help?

Hello, I looked for this on the forum but did not see anyone referencing this problem. if I miss it, I am sorry.

Recently I could be on SL for any given amount of time, which ranges from 2 min all the way to 30 or so, before everything just FREEZES on my screen. I have frozen SL screen, and frozen window bars below, but no response from anything. Only a hard reboot wakes the computer back up. I don't get any blue screens or errors.. but a few times after restarting I have gotten active desktop errors, saying its been shut off due to a windows crash.

I looked in my event logs for any errors that might have occured before this happened, and there is nothing. My computer is not running hot at all, from what i can tell. all the fans seem to be working and it's not hot in the box whatsoever. This just started yesterday. Before I could be on SL alllll day with no trouble. Can anyone offer advice? Comp specs are as such:

AMD Athalon 64 3700+ 2.41
2.00 GB DDR crucial ram
Nvidia GeForce 7800 GTX
60 GIG WDC HD

any advice is such appreciated. Thank you!

Edit: Just want to add what I tried already. I did a virus/spyware scan, along with a scandsk and defrag check. Everything is free and clear. I also updated video and sound drivers, and reinstalled SL.

These symptoms sound like an under-current feed from the power supply to the motherboard.

Even if nothing had changed anywhere, components degrade over time and start drawing more power, or stop supplying the same current from the power supply.

Unfortunately, this is kinda hard to diagnose without lab bench equipment. An easier test is to replace the power supply. With a 7800 card in there, you may have been riding the hairy edge already.