If you have a boot disk that has FDisk (which is getting more and more rare nowadays since it is Windows95/98 technology) an easier way to do it is to boot to that disk and run fdisk /MBR

That will format ONLY the Master Boot Record, and leave the already installed Windows intact, at which point the Windows install boots normally.

A better bet, however, would be to download Knoppix, and follow the procedure here under the section "Kill the MBR" then reinstall Windows. I will caution anyone reading this, though, that modifying your MBR most times will lead to you having to re-install windows on that drive, so only do this if you are prepared for a reinstall.