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 -Originally Posted by David75
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.