Just formatting the drive does not delete the information It deletes the "table" if you will, but the actual contents are still there.
If you want to delete data you need to write over it. There are MANY freeware tools out there that will LLF a drive.
Ultimate Boot CD is a great live cd to get started with and parted magic has some amazing tools but if your super paranoid then feel free to use multiple LLF (Low Level Format) programs and run them a few times each.
Once you think you've gotten rid of everything, use some of the tools to recover the drive to see if you really have "deleted" the data.
Side Note: Government LLF's write 30+ times a variety of passes of 0s then 1s etc etc to scramble as much data as possible.
If your super paranoid you can always build Degaussing coils and go to town with them, then run the platters through an industrial shredder, melt them down etc etc