Answer:
It can be caused by software or hardware error. To fix software failture you have to boo up in the Safe mode anf clean your registry especially keys Run also you can use utility which is called msconfig.exe (Start->Run->msconfig). Check the start up list.
If it's because of hardware you have find a special CD which will allow you to boot the system and run tests.
PS One of the most common reasons for pcs to freeze is the CPU high temperature. To fix it you have purchase new CPU thermo paste and replace the old one. How to replace thermo paste you can find in Google.