Assuming that you want to clear off the entire drive and have no concerns about losing any of the data that is on it, boot Linux and enter
dd if=/dev/zero of=/dev/hda bs=512 count=1
NOTE: this will completely wipe your entire Master boot Record! Not only will you not be able to boot from this drive, you will not even be able to read it once you've rebooted: it will require partitioning (via fdisk) and formatting. Be sure that you have your new OSs boot floppy close to hand!
If you wish, you can partition the disk while still in Linux (wiping out the MBR only makes itself felt after you reboot); "fdisk" and "cfdisk" will happily write DOS partitions to your drive.
If you have to do this from DOS, try entering the following commands:
lock c:
fdisk/mbr
Several folks have reported having no luck with "fdisk /mbr" - which, in my long experience with it (I discovered it by reading the strings in "command.com" in DOS5.0 well before someone documented it as an "undocumented feature") both succeeds and fails silently. I believe that in the case of Windows-based installs, this is due to the fact that Windows disallows "raw writes" to the disk: a useful thing to prevent a certain percentage of viruses, but a hindrance when you need to fix something. "lock c:", above, "locks" the drive to allow them, and should make "fdisk/mbr" effective.
