The story so far....

I've built the following computer:
Gigabyte 8IHXP Mobo
p4 2.4 533 FSB
512 MB PC-800 RDRAM

and the CPU wouldn't work. I took my cpu from another computer:
p4 2.6 w/ HT 800 FSB

and put it in, and it worked fine. I ordered a replacement CPU, and am having the same problem.

I suppose it's possible that the replacement CPU is also bad, but could there be something else going on?

*********SOLVED**********

It was a cache issue, the mobo doesn't support the 1MB cache CPU