You need to boot the PC without loading Windows. So you need to ensure that it boots from a drive other than your hard drive. The order your PC looks for a boot disk is determined by the settings within the BIOS. It will probably be set to look at either your floppy drive or your CD drive before it tries to find the boot information on drive C.
If you've got an original Windows OS disk, try putting it into your CD drive and rebooting the PC. If the BIOS is set to give priority to the CD drive, your Windows disk will be recognised as a boot disk and you'll be taken to the command prompt. If it doesn't work, go into the BIOS to check that your PC has the CD drive set as the first boot option. (To enter the BIOS, follow the on-screen instruction at boot-up. That usually means pressing F2, F8 or Del).
If you've not got an original Windows disk, or if you find that you can't boot from it, use a boot floppy instead. (You'll need to check that the BIOS is set to boot from the floppy drive ahead of the hard drive). You can use a Windows 98 or ME disk, which you can download from here:
http://www.bootdisk.com/bootdisk.htm
(Keep the floppy in the drive and restart your PC to get to the command prompt).
Chris