hl2 runs fine on my Windows 7 System =P

That being said -- Zero does have a point. It may be your GPU. It doesn't exactly sound like a power supply issue unless it's an older PSU and the rail that is feeding power to your GPU is faulty or old. I would try running it on your integrated card (simple as plugging your monitor into the onboard port and changing the options) and seeing if the same issue happens. If it doesn't, then it's your graphics card or PSU rail leading to the graphics card. If it does, then it's something else.