have you got a start up disk(floppy), if you havnt you can get this from add and remove programs and create start up disk...once you have a start up disk, restart the computer with the floppy in the a: drive, and you will come to start the computer with cd rom support and start the computer without cd rom support chose either, and then you will come to a command prompt which looks like this:
A:
now type format c:
so it looks like
A:format c: