Lag problems

[gp20789]

I am not as tech savvy as I wish to be so I don't know a whole lot. I have recently been getting random lag spikes while gaming. My computer should be able to take what I put it through as far as I know. I play COD: Modern Warfare and guildwars, both on highest graphics settings. How do I make the lag spikes go away, or just be reduced. Also, I recently had to get a new video card put into my computer. I got an ATI Radeon HD 4850, which has 1 G of DDR3. And when I got it back from Best Buy everything was working great. Then one day while playing COD in game my computer shuts off on its own. Thinking it may be a virus I ran a virus scan that came up with nothing. I then tried playing COD again and after about 10 minutes or less of playing it shut off again. I then uninstalled COD fully thinking it may have something to do with the game since it didn't happen while playing Guild Wars. The random shut offs stopped, but the lag spikes continued on guild wars. I would have excellent connection speed, since I play wireless. I even went out and bought a new wireless adapter just in case but I still get the spikes. Then just recently my computer shut off while playing Guild Wars.

What do I do?

Here are my system specs
Mother board- ASUSTeK Computer INC. P5N-E SLI 1.XX
Operating system- Windows Vista Ultimate Service Pack 2 (build 6002) 32-bit
Processor- 2.67 gigahertz Intel Core 2 Duo
RAM- 2.00 GB
Brand- Velocity Micro
Model- Custom PC
My C:/ Drive has 252 GB free of 372 GB
Bus Clock: 266 megahertz
BIOS: Phoenix Technologies, LTD ASUS P5N-E SLI ACPI 0602 Velocity Micro Edition 04/25/2007

[Aus_Karlos]

Hi gp20789.

Could you please post your PSU specs, also can you download Everest from my sig to check temperatures.
After about 5mins of gaming minimize and post your temperatures.