I just got a new harddrive for my MacBook about a month ago - I partitioned it and installed XP on one partition and 10.5 on the other partition. After using it for about a month, it stopped working.
I think that what happened was I left the computer on and the battery abruptly died without going into sleep mode. Anyway when I turn the computer on the Apple image comes on with the spinning image below it. It will stay on this screen for about 60 seconds and then restart, chime again, and then do this process over and over again. After about 4 tries it will go to a flashing hard drive with a question mark. When I try to boot into XP it freezes up, I see the blue screen of death for a second and then turns back off.
I booted off the OS X DVD and did a verify and repair and it did not find any errors but it also would not mount the hard drive (it was greyed out).
I now put my old hard drive back in the MacBook and it is working fine and I ordered an external enclosure to see if I could mount the hard drive that way. Also, when I gently shake the hard drive I hear a little clicking sound but it doesn't sound serious like there are broken parts inside the case.