This happend to me also. You didn't say anything about BSD or Stop Codes.
Here is what to try if you haven't already:
1. As your computer starts, hit <F8> and get to the boot menu, then try
selecting "last known good" (these selections are parapharased)
2. If it still reboots, try <F8> again and boot in safe mode.
3. If it still reboots, try running the Recovery Console from your Reinstall
CD. If you can get to the Recovery Console, run CHKDSK /R
4. If you can't get to the Recovery Console because your computer "hangs"
then try to get to Setup and do a Repair Install.
5. If Setup hangs, then your ntfs volume is bad, either from software
(which is what happened to me) or hardware glitch. You will want to run
hardware diagnostics on you processor, memory, and HD as a minimum. They
were all fine for me and I ultimately had to reformat the HD and reinstall
from scratch. I have not had problems since, but am somewhat leary of ntfs.
There are tools on BartsPE and WinUBCD that it creates that may help, but
you need to have created that CD yourself already. Good luck!