Ages after the question I know but the short answer is that you can't.
Even the Gutmann algorithms (write patterns over the disk 35 times) are not sufficient to negate modern tunnelling microscopy technology. Unless you smash the disk to pieces or wipe it using a degaussing coil (a damn big coil at that!) then forget it - govt level agencies (and/or multinationals) have the ability to retrieve the data. I'm not kidding and none of this is new. Forget DoD standards (7 pass wipe?) as even I can get data back using software after that.
PS - do remember that in most cases "wiping the disk" in a Windows environment really means caching data in memory (either system RAM or the drive cache). It is remarkably difficult to make Windows XP (Vista is worse) write to disk PROPERLY AND WITHOUT CACHING in a reliable. repeatable manner.
Bestcrypt works for me and has done for years. It does cost though (its cheap) so many people go for Truecrypt. While Truecrypt has source code available for review it doesn't appear to have the encryption algorithms I'd wish - and it still persists in saying SHA1 is secure (any encryption when proven to be susceptable to non-brute force algorithms is dodgy - as SHA1 was a couple of years ago). I wouldn't touch Truecrypt based on their "assessment" of the state of play in the secure hash "market".
It occurs to me that hash probably means something you smoke to most of you?
A hash is a one-way (supposedly) mathematical function - eg you could hash your name and in theory nobody could take the hash output and get your name from that. In theory.
