1) First think, which software you installed on your computer last.
2) Safemode - You need to go with safemode, power on your computer and the OS loading time (after displaying bios menu) press F8 key continuously.
In the next screen select safemode.
After selecting Safemode you will see like below image
3) Select "System Resote" (in Xp start=> accessories =>System tools=>System Restore
4) Select "Restore my computer to an earlier time" and select one of your last installed program and click"Next"
5) Your computer will restart and some process
6) After the restoring process, your system restart and boot normally, but it needs more time, After that you will see the message " Restoration Complete".
7) If your system restore successfully complete, your system Restart problem was solved. If not, you should
need to reformatting your system