computer is not getting switched on.the display shows random colours after sometime.what could be the problem?

Is anything powering up? Do you see the POST (Power ON Self Test)? Have you tried to boot to Safe Mode?

We also need more info about the machine itself, Brand, windows version etc.

yes it boots up until the logo of xp appears, after that the screen shows random various colours.

OK, that sounds like a video driver problem. Try booting to Safe Mode and make sure the right video drivers are installed.