OK, even a "slow format" only reads the entire data area of the drive to find any errors so it can map them out so formatting is nondestructive to the data.
On the other hand, unless law enforcement wants your data so badly they are willing to spend a lot of money to get it, a single pass of any utility which writes data to every sector renders it irrecoverable. Active@KillDisk has a downloadable, free, floppy or ISO image which you can then boot to and truly erase everything: http://www.killdisk.com/downloadfree.htm
For those of you who may be interested in the technical details, here goes..... Every time you write on magnetic media, both the width of the magnetic flux changes and the patterns are different. The result is that, when new data is written, very narrow bands of the old data remain on either side of the new write. If any of you have had a VHS or cassette tape degrade in quality because you reused it several time, this is why that happened; the old recording is mostly; but not completely, gone. On hard drives, this phenomenon still occurs so the government developed electronics that could cause the heads to read on either side of the main data tracks. This is not trivial and no mere mortal is likely to possess the technology to do so. You have to completely remove the drives logic PCA, replacing it with the "secret" control electronics, which can then be controlled to locate those bands of residual data and; hopefully, make sense of them. This is up there with other spy gizzmos like the one they can point at your office window and read what is being displayed on your CRT because the electron beam actually vibrates the window. (This gizmo does exist; but, is probably useless on an LCD).
The point is, if you wipe the disk with something like KillDisk, you are completely safe (unless you happen to be a kingpin in a drug cartel). On the other hand, I have personally recovered thousands of peoples files after they formatted and reinstalled Windoze and have been floored by what I found on drives I purchased on EBay (like tax returns!).