Computer keeps on restarting by itself

I have this problem with my other computer. It has Windows Xp Professional installed. When i turn it on, this message comes up, "We are sorry for the inconvenience Windows did not start successfully". Below the error message are some reasons why this could have happened, it says it might have happened because of a power failure. But i think it's a virus or something like that. Below that are 5 options for me choose to start the computer, Safe Mode, Safe Mode with Command prompt, Safe Mode with Networking, Start Windows Normally and Start using the computer with the last known good configuration. No matter which option i select, the computer just keeps on restarting by itself again and again. Also, i lost my Windows Xp CD, so i can't reinstall it using that. Does any one know how i can fix it?
Please help
Thanks