Computer keeps restarting

Yesterday I installed an update and everything seemed to be working fine. Now, once I boot up the CPU, it shuts down and restarts itself. I tried booting it in safe mode but it does exactly the same thing. There are no errors that pop up. The closest I got to logging in was the login screen (With password) but, I don't even have a windows password. Help?

whatever u r using XP, Vista, 7, just put the CD in the CD-rom and boot up
then choose repaire, that should fix the problem.

its ur heatsink it may be cloged you need to clean out ur comp by opening it