If you want to just format it but don't want to reinstall another Windows, you can do the following (there are other ways) :
1) In some other machine (unless you have yet active your Windows, would be better there) make a bootable CD or DVD. You can do it with many tools, one that comes to mind is CDBurnerxp (there are many other solutions, like BartPE)
2) Ideally copy the needed console commands to that CD. That is, the files for format, and probably fdisk. They're found at your /system32 folder of your windows. If not possible, copy them to some device you will be able to read from console. (those great old floppy disks...)
3) from the device where you have these files, type format c:
You have a list of console commands in Windows here.
If you have a sort of not totally devastated installation, you yet might have the ability to hit F8 key when starting the machine(hit it several times once you here the "bip" of your machine when booting). There you can choose to boot in console mode (can't remember exactly how it calls it) . If you had copied the format etc files to some device (ie, floppy, usb stick if booted that way and have it available at your bios) you can type that unit (ie, a: , intro) , then type format c: and follow instructions.
With just a windows CD is a matter of boot in repair mode and use the command...
edit: sorry, crossposted with RubyWedge.