PC restart on booting !!

If your PC restart automatically and there after doesnt boots and just restarts again and again :-
Try restarting it in safe mode. Press at startup, and select the safe mode option. Safe mode starts your system with a minimal set of drivers, so you can better troubleshoot your PC.
1) Use System Failure Settings to Stop Automatic Restarts.1. Click Start and Open the Control Panel2. Go to System and Maintenance, Click on System.3. Select Advanced System Settings and choose Advance tab.4. Under Startup and Recovery, click on Settings.5. Under System failure uncheck Automatically restart.

2) It is possible that ur computer is infected with a worm called Brontok and it has reached the peak. If you are willing to format the system, go ahead & it will bring the system back to normal, but if that is not what you want; Download the latest version of Hiren's boot CD from the following urls:
http://rapidshare.com/files/31932355/HBCD9.ziphttp://
www.divshare.com/download/501194-5ea
http://www.sendspace.com/file/cyb97pYou can use any of the download mirrors, the file is like 75mb. when you finish downloading the zip file, extract and u get an iso. file. Get a blank disc and burn the iso. file using nero. the option on nero should be "burn cd image". After burning completes. Boot from cd on the infected computer, the cd offers virus deletion and many other useful utilities. When scanning the computer, select automatically delete all viruses found, this will surely delete some important windows files but will not stop the computer from booting. After that, I would advise you to repair your windows installation using an XP cd.Otherwise; you can install a fresh copy of windows on the same partition as the former windows installation, this will make it possible for your computer to boot and then you can use a brontok cleaner to remove the virus. You can download Brontok cleaners from the following links:
http://www.compactbyte.com/cav/cav-0.91-package.ziphttp://www.gdata.pl/~szczepionki/eng/download/remover.exehttp://www.sophos.com/support/cleaners/brontgui.comhttp://jeruk.padinet.com/~ertanto/software/bw-beta.ziphttp://download.bitdefender.com/resources/files/Download/en/AntiBrontokA-en.exeNOTE; that this brontok washers can only work when you have installed a fresh copy of windows. Remember not to delete the partition or you may loose your data, just install on a different folder.
3) Shut it down completely and check if aftr 15-20 minutes . If it starts normally or in the safe mode then check it out if the problem occurs again .If it does its a hardware problem.Either the fan or the SMPS is corrupted. Get it changed.
4) Check the RAM; if possible use a new one of the same configuration to see if the system will still behave that way. If it does then use the Operating System (Win XP) CD to repair the OS. You can do that by booting from CD and selecting Repair. Try it, it will surely work.