I always recommend a reformat of your hard drive once a year, twice if you use it to earn your living, as with Affiliate Marketing. And, it goes without saying that you diligently back your computer up, right?....
But reformatting a hard drive simply isn't enough to guarantee that you will eliminate data. Old files consisting of emails, mp3's, pictures and financial data and passwords are recoverable with simple to use software. That means that if you sell your computer or donate it to charity, the new owner could retrieve all of your old data that you had "deleted".
Even an old hard drive that no longer works should be physically dismantled as the platters inside could be read if someone wanted to badly enough. This involves cleanroom procedures and money but I still tend to err on the side of caution.
Anyway, if you'd like to use some software that meets the ridiculously stringent specifications of government agencies, you can try Remo Shredder.