if you have only one mobo that's it tough to find out, and I am sure you've tried a few things to shoot at, but PC has only 10 parts at the most, and to find out what is coasing the problem is not that difficult if you have something to swap it with, if you don't, well that's kind of a dilema to deal with. In yer case it can be HD, cable, memory, or the IDE controller on the mobo (did you swap IDE1 with IDE2).
And one more thing, you have to be more specific when you say something like "reboots" - when, how, etc. It is hard to have a specific answer to a general question. I wish you'll find what is the problem in your PC, I can't trobleshoot from guessing.
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