Use a working computer, go to Ubuntu.com, and Order an Ubuntu installation CD. Linux is safer, faster, more intuitive, open source, free, etc.

If you're really insistent on having Windows or not installing a new OS, I'd try going to the Windows homepage and explaining the problem to customer support. If that still doesn't work, you could always buy Windows (I think XP - 7 should still be available) at your local Best Buy, Staples, etc. although I believe they're a few hundred dollars.

Also, check the box the computer came in. More often than not, a re installation CD is included.

Hope that helped.