In reply to JamButty:
If you just delete all files, or do a format, then recovering the files would take me an hour or two (depending on disk size). Comercial recovery £10-£30.
If you break up the chippery on the drive, but not the platters, recovery is in the £400 region. I could maybe do it myself, but it would take days of careful work and buying some special parts/tools.
If you break the platters into coin-sized bits, recovery needs a high-spec research quality microscope. Cost will be many thousands. Only a few government labs are thought to have done this successfully.
If you break the platters into a fine powder (0.01mm grains), melt them down etc. the data is unrecoverable.
If you overwrite the data once, with random data (by using one of the above mentioned tools in single pass mode), it may or may not be possible to recover some of the data. It would need a research quality microscope even better than the one mentioned above, and the success rate would depend on the age and type of drive. As far as I know this has never been done, and the cost would likely be 5 or 6 figures.
If you overwrite the data with several passes of random data (using one of the mentioned tools in multi-pass mode) the data is unrecoverable. There may be a few scraps of data left in areas of the drive that are damaged and therefore automatically left unused, but they won't amount to much.
If your drive is new enought to support ATA Secure Erase, that is very effective. If your drive is from a win98 machine, it's probably too old.
In your case, just formatting the drive is probably adequate. But if you want peace of mind use one of the tools mentioned above. DBAN is my tool of choice. Despite the disclaimers on its homepage (put there to try and sell blancco's commercial offering) it is simple and effective.
Post edited at 21:57