I am not able to shutdown my computer , my system get restart when i click the Turnoff computer.

Start here: Windows XP Shutdown Troubleshooter

Right click on COMPUTER, select PROPRTIES, ADVANCED SYSTEM SETTINGS and under STARTUP and RECOVERY select SETTINGS. untick automatically restart. That should allow you to see the error. Also look in the event log. The event viewer is in Control Panel->Administrative Tools->Event Viewer->Windows Logs->System Log