Had a weird problem with a PC - it was all running perfectly normally, then one day wouldn't start. Starting in safe mode printed out all the drivers until it gets to the line ending in mup.sys, apparently something to do with USB drivers.
After that line the screen flashes a couple of times, and the PC reboots. Repeat. Forever.
I spent some time reading of the gazillions of people who have had the same issue. Some have tried changing disks, RAM, PSUs, reinstalling operating system, USB drivers, everything conceivable about the computer.
Then I stumbled on this post:
http://cyberiapc.com/forums/index.php?showtopic=5288&st=15&p=33885&#entry33885 in some random forum. Try "chkdsk". Just before this I had run fixboot, which hadn't really made a difference. chkdsk had to be run with a force, but reported a few errors.
Upon reboot, the machine started normally. Great, thanks OP!!