Look for other methods first; but if it comes to last-resort methods:

If you have a "known brand" hard drive, and it's not completly obsolete, try going to the HD manufacturer's website and getting their setup/diagnostic utility. You'll probably find it somewhere in their support area.

While there, you may want to poke around a bit to find out and understand the difference between a normal format and a "low level format." New hard drives nearly always come with a low level format done on them, and the manufacturers recommend that you DO NOT EVER DO A LOW LEVEL FORMAT ON THEM. (then they add "!!!!!!!!!!!!!") But many of the Diagnostic/Setup programs are capable of doing it if you must.

This format should be able to wipe everything, lays new tracks and sector markers, etc., so that it should give you a totally clean disk. It's not possible(?) on some drives and on large drives that use overlay methods for getting the extra addresses it may lose considerable drive space; but if you must...

Be sure to rewrite the SYSTEM when you do the normal format, to hopefully get rid of anything in the boot sector. Even a low level format doesn't always wipe there, and rewriting a new SYS won't necessarily clear an "overrunn" boot sector worm; but there's a pretty good chance it will. I don't know of a program that guarantees a clean boot sector, although some of them may clean it up without mentioning it in the specs.

Really old Norton Utilities had general purpose stuff for doing this sort of thing, but if they're old enough to have this utility they're probably too old to work with current Hard Drives (over 10 GB perhaps). I don't know what's in current Norton SytemWorks, since I haven't really needed to look. WinXP has a built-in secure-wipe, although I don't know how good it is. You may find an ISO-Secure diskwipe utility in current or recent Norton or similar - as I said, I don't know whats there.

Note that if you're working with salvaged drives with unknown content, you should ideally put the diagnostic/setup software on a write-protected floppy or burn it to a CD and use it in a ROM that can't move crap onto the setup program source. If you're paranoid, you'll want to disconnect any other writeable drives until you've got the new one clean. Rumor is that lots of HDs get "recycled" because the original owner couldn't figure out how to get rid of the malware. (But it's just a rumor.)

Personally, I'd pop for a new drive that I know is clean; but suit your own needs of course.

(Running a magnet over the HD may corrupt the disk drive motor magnets, and may damage the read/write element, among other things.)

John