Originally Posted by cboucher
If you just format the drive and write to it (reinstalling the OS, your apps and your personal files should do this sufficiently) you should be ok.
Unfortunately this is not true.
If you really want to wipe the data from a drive without any chance of recovery, it should be wiped with a low level formatter at least once, twice if you're paranoid and three times if you work for the NSA
Darik's Boot & Nuke is great for something like this, and it's VERY thorough. Darik's Boot And Nuke | Hard Drive Disk Wipe and Data Clearing
and believe it or not, smashing a hard drive doesn't render the data completely irretrievable. By most folks? Definitely plausible, but if someone REALLY wants to know this information, they'll get to it.