I just had a computer yesterday which was a bitch. Similar symptoms to yours, got stuck in a re-boot cycle, the guy who owned it reported that the boot time had got longer and longer until it wouldn't boot at all. Tried repairing the MBR, the boot sector and everything. Windows reported the partition as "unknown", although I could still access some of the data by booting from a Read NTFS floppy (
www.ntfs.com I think).
I tried to re-partition the hard drive, format and install winXP, but the install wouldn't complete. After 3 hrs of formatting the hd, it got to 17% and then gave up. FDISK failed on the "checking drive integrity" part......
Turned out to be the hard disk. A new one fixed it.
EA