I've been seeing this problem cropping up quite often and no definitive answer either.
The first thing that occurred to me was it's due to a corrupt user profile. It can't logon to that profile so it reboots.
Here's what I'd try if I were working on a computer with this problem.
Boot into Safe Mode. To do that, repeatedly press the F8 key during the boot process (don't hold it down because that'll probably give you a keyboard error).
You should get to a boot menu where you can choose Safe Mode and then press the Enter key.
You will be given the option of logging onto the Administrator account. Click on it and if you didn't put in a password for that account (most people don't), leave the password blank and press the Enter key.
If it boots on into Safe Mode, go to Control Panel>User Accounts and set up a new user account with administrative privileges, then reboot the usual way and logon to the new account.
If that works, you can
click HERE for instructions on how to copy the data from the old account to the new. Once that's done, just delete the old account.
Please post back with the results.