I sold a computer last year. I do my personal finances on Quicken, and have other personal information on the computer.
CCLeaner did the trick for me. It took a couple hours, but after I "deleted" the sensitive material it (supposedly*) overwrote all the "empty" sectors of the HD with ones, and then with zeros, five times each (IIRC). If there was any accidental donkey pr0n left over (I don't want to talk about it) I'me reasonably certain it was lost forever.
*In the ensuing year i have had no troubles I would associate with someone getting my private info. This tells me (a) the person who bought my computer didn't look for it, (b) wasn't knowledgable enough to find it, or (c) the software did what it claimed to do.