I frequently use a program called wipe.exe.
You can download the program at
http://www.ibm.com. Use the search engine on the site to find the program.
This program almost always allows me to clean a hard drive when fdisk jsut doesnt seem to cut it.
Simply boot from a floopy, then run the wipe.exe while at DOS prompt. Once finished, then do your typcial fdisk routine and you should be fine. The wipe program itself will tell you the correct command line arguments when you try to run it.
Hope this helps.