Are you using Windows XP or Windows 95/98/ME?
You also made a post at:
http://www.softwaretipsandtricks.com...?threadid=5188 Please don't make multiple posts.
Without posting error message its impossible to find out why your computer is randomly crashing or rebooting. There can be 100's of reasons.
Go to System Properties > Under Startup and Recovery Settings > Un-tick Automatically restart under System failure.
Then when you computer crashes or reboots go to Control Panel > Performance and Maintenance > Administrative Tools > Event Viewer then copy & paste the error message here.