Most annoying problem. Ever. You're just working happily along when all of a sudden BAM, the Blue Screen of Death pops up and "IRQL_NOT_LESS_OR_EQUAL" very prominently appears. Generally with a cutesy little hex error towards the bottom of the screen. STOP: 0x0000000a, or something like it.
Sadly enough, every time I've run into it it's been a hardware error. Most commonly it is the CPU overheating. A close second would be bad RAM. The first thing I would do if you have this issue is to run the machine till it gives you that error, then immediately reboot it, go into the BIOS and check the CPU temperature. 3/5 times it will be way too hot. Go to the store, buy a fancy new heatsink with a ball bearing fan (don't need it binding like a bushing fan will eventually do), pick up some thermal grease while you're at it, and install. Your problem will likely go away.
If not, get some memory that you know is good, and test that out in the system for a while. Be sure to pull out all memory that is currently in the system, or this test will do nothing. If it stabilizes after that, then replace the memory permanently, shoot the old like a clay pigeon and have a nice day.
Sometimes though, especially with the IRQL_NOT_LESS_OR_EQUAL error, even that is not enough. This is where you get hardcore with the troubleshooting. In the BIOS, load Fail-Safe defaults, disable the onboard modem, sound, and LAN (where applicable). Take out all the cards, with the exception of video, and run the machine for a while. If the machine stays stable, then start adding/re-enabling the devices one by one. Keep in mind that modern motherboards share IRQ's across PCI slots, so try not to put a sound card and a modem right next to each other. They don't seem to think that's funny every once and a while. Anyway, do this one by one until you start crashing again. Whatever you put in last is the culprit.
Also, many newer motherboards' northbridges are actively cooled. Almost always by a bushing fan. Bushing fans tend to bind after a while. Since this chip controls the data stream between the RAM, video, and CPU, I'd say it's pretty important. If it's overheating bad things are bound to happen.