If you're booting from another hard drive (ie. your old hd is just sitting there plugged in but doing nothing), there are some programs you can get that wipe data, rather than just "delete" it or format it. Like, it goes through and rewrites 0s, 1s, and other random patterns over the entire drive as many times as you want. I've never actually attempted to recover any data after using that, but they say they are foolproof even from hardware data recovery methods (like going in and looking at the magnetic remnants on the platter). Though Extrm Bob's post makes me wonder about that...
Anyway, one of the big ones is Norton WipeInfo, comes with SystemWorks. There are also several smaller ones you can find on Download.com, just search for something like "file wipe" and it ought to give you some good results for it.
---Jamin
