PC is always crashing

Hi. One of my WinXP computer's is always crashing. When you turn it on it will show a blue screen with an error of
"IRQL_NOT_LESS_OR_EQUAL"
and sometimes its
"SPECIAL_POOL_DETECTED_MEMORY_CORRUPTION".
If you restart it, sometimes it will boot but after about 5 or 10 minutes it will show 1 of the error messages.
I thought it was a driver problem but it even does it in msconfig's "diagnostic startup mode" and also in "safe mode".
I think it might be a hardware problem because I ran AVG, Spybot and some other antivirus' and spyware removers and they found nothing.
I also did a Windows XP repair and it ran good for about 2 Hours, but then the same thing start to happen.
Any help is greatly appreciated.

Looks like a bad memory chip. Run a diagnostic on it and you will know for sure. Is it a dell?

Thanks for the responce
No It is not a Dell.
It is a homemade job.
I don't have a clue who makes the mobo?

What diagnostic? memtest86?

yeah that might work. but the reason i asked is cuz dell makes a nice system utility you can use to diagnose hardware problems. Anyway, how many cards you have installed? I would try removing one and see if it works better. If it crashes, swap them and try again. At least you would know which card is bad at that point. Or if they both crash, then it very could be the mb.

The PC has become so bad now it will not even boot. It has three card in it and I removed the modem and sound card. Obviously, I had to leave the video. I also removed both cd and DVD burner form the system.
I wanted to try it with just the HDD but it still does not boot.
I tried the Mem86 and it said the memory was bad.
I was also able to try the PC's memory in another computer and it did not work so the RAM must be bad.
But the funny thing is, I tried good memory in this PC and it still will not boot.
So there must be something wrong with more than just the memory.
Any ideas?
Thanks for your help.

try resettiong BOIS or getting a new hard drive,

did you overclock anything?

try your hd in another computer see if it boots

Well if it's bad ram, could you replace the current ram with other RAM?