Start > Run and type in msconfig
Select "Diagnostic Startup" click ok and reboot.
**this will boot your PC in normal boot, but since all services and startups are disabled it will be kind of like safe mode.**
If your reboot is successfull and your pc is now stable, go back into msconfig and go to the Services tab and select one checkbox. Click Ok and reboot again.
**this will take a while, but you can find out the offending programs or whatever that is causing this startup.**
For every sucessfull reboot, check another box and continue untill your PC becomes unstable again. Uncheck that one and reboot to confirm, then continue with the next check box after that, noting which service/startup entry is causing the instability. Do the same for the startup tab as well. There could be more than one service(s)/startup entries causing the instability.
once you have that list of processes that are causing your trouble, google its name and see what they belong to, and if its a certain program you have, if there are fixes to it etc.