My computer randomly restarts when playing games. Halp

I have had my computer for over a year now. It started crashing within a few months of getting it. I did a reinstall of the graphics drivers by using driver sweeper first. This got rid of the problem but a few months later it started happening again. I updated the BIOS after this and again, it solved the problem. Recently however, it has started again and I can't find anything that will fix it. Crashes generally happen when playing games like SC2 and ArmA 2 and a BSOD screen isn't given on reboot - I have auto reboot unchecked.

One extra thing to note: I recently removed my sound card and its drivers. Before this, the crashes were accompanied with a loud buzzing noise. Afterwards, it seems like the computer is just performing a regular restart, when it shouldn't be of course.

SPECS: Processor (CPU) AMD PHENOM II X4 QUAD CORE 970 (3.50GHz/8MB CACHE/AM3)
Motherboard ASUS® M5A78L-M/USB3: M-ATX, USB 3.0, SATA 3.0Gb/s
Memory (RAM) 4GB SAMSUNG DDR3 DUAL-DDR3 1333MHz (2 X 2GB)
Graphics Card 1GB NVIDIA GEFORCE GTX 550 Ti - DVI,HDMI,VGA - 3D Vision Ready
Memory - 1st Hard Disk 1TB SERIAL ATA 3-Gb/s HARD DRIVE WITH 16MB CACHE (7,200rpm)
1st DVD/BLU-RAY Drive 24x DUAL LAYER DVD WRITER ±R/±RW/RAM
Power Supply 450W Quiet 80 PLUS Dual Rail PSU + 120mm Case Fan
Processor Cooling STANDARD AMD CPU COOLER
Sound Card Sound Blaster® Audigy™ SE (since removed)

The minidump file says it was caused by driver: ntoskrnl.exe.

Any help would be much appriciated!