Quote Originally Posted by David75
First:
Check Connections. Screen/Video Card and Video card in slot.
Check power connections for the video card and mother board. You get my point, if you can check all of these little things, do it

Nvidia card? Nvidia nforce? drivers and bios for the latter.

next is the harddrive... scandisk or better tool to seek for bad sectors.

I doubt the ram could be a problem, a memtest could tell more.

Back to square 1 with a different path: is the PSU ok? hard to tell most of the time. maybe with a cpu+gpu burn you could tell.
Few things to add: (and yes please, do it in the order listed) is before checking the power connections from the PSU to the mainboard/video card -

Excessive dust? Aberrant case screw? Grab a can of compressed air and/or point the open part of the case to the floor and give it a good shake. Then check the power connections from the PSU to the other various parts of the PC.

Then after checking the mainboard BIOS and driver updates, read up on your PSU and video card to make sure you've got the right amount of juice for your devices.

One more thing to consider is the answer to the following question:

"Are you a static electricity magnet?" It could simply be you're flooding you PC with static electricity when you reach down to grab that USB stick. Try "discharging" yourself first by placing your hand on the computer case.

I've been having that kind of problem lately. I green screened my work's vista machine because of static electricity. Zapped several others over the course of this winter season creating dark screens with green lights. Christ I even powered on a machine with static electricity.

as always, good luck and report back.