"But even overwriting the disk with something else or formatting it does not guarantee that the sensitive data is completely unrecoverable. To deal with this, there are programs that write random data to the target regions on the disk many times over and over, so making data recovery unlikely."
I used to own a piece of software called System Mechanic that did all of this, and it didn't even require an intense load on the CPU or hard drive.
If you delete files, you're safe from 99.99% of people.
If you overwrite the files once, you're safe from all but a handful of people.
Overwriting once is generally good enough, unless you expect that people may want to spend hundreds of thousands (or more) to recover your data.
In any case, you still have to be careful if you're only deleting files (not the entire harddrive). Files may be stores in more than one location (if they've been moved in a defragmentation), or data from the file may have been written to the pagefile at some point, which can still be present. The only way to really be sure about that is to delete the whole drive, or, better yet, destroy the drive (into dust).
