As long as you can plug it into a functioning computer with an external enclosure or a USB adapter, I would use the new wipe hard drive feature on
CCleaner 3.0. It will do up to 35 passes and have the option to just wipe empty drive space or all drive space. The empty drive space option is good if you really need to get rid of that incriminating porn from the hard drive before the tech savvy wife finds it.
If it is just a self contained computer that you want to wipe the drive, use DBAN as suggested in the previous post. It only does 3 passes and takes longer than CCleaner, but if 3 passes is good enough for the Department of Defense then it is good enough for me.