Yep, DoG is right. It's a bad hard drive. I have a Maxtor 40 gig, that does the exact same thing. But, if I leave the computer running for about 3 minutes, at the boot screen, and then hit the reset button it works. You can actually hear the hard drive start up, WAYYY after the computer has.
The cause for mine to start acting up was because I smacked it a couple of times...it was making noises, and I wanted them to stop. And guess what! They did!