I'm a little bit of a security freak with old hard drives.  I have 2 portables and a desktop I bought at yard sales. Plus several others I have purchased at yard sales or used over the years that I no longer use. There is only one way to make sure the data is secure.  Unplug the computer and open the case (how depends on the box.  May be a no-tools operation or may take a screwdriver) Find and physically remove the hard drive.  Take it to the shop and cut it in half with an acetylene torch.

As an alternative hits with a 6 pound sledge is usually effective.

It's unbelievable what folks leave on their computer and I'm told it is impossible to clean it.  The used computers I mentioned have had credit card information, personal ID information, photographs, letters, resume's, videos ... a LOT of stuff I am certain folks did not intend to let someone else have access to. 

I spent a couple days in fire officer training.  That included a few hours with the State Fire Marshal's chief counsel.  She told of a case where the officer was using a Fire Dept computer for things including personal web surfing.  He realized they were on to him and used a professional drive cleaning program to wipe the hard drive.  When the techs got done they printed out copies of everything he had downloaded.  She was amazed at what they recovered.

So I guess the answer depends on  what is on your computer.  I have one hard drive that will never leave my possession alive. It is in a stand alone computer and has financial and farm records on it. The laptops I don't put very much sensitive info on.  But I have ordered things online with them before, so they potentially have credit card info.  They have passwords. They have personal information. They probably will leave here in pieces.

If you are certain there is nothing sensitive on them a cleanup program might be OK. Kim Kommando recommends Eraser ( http://eraser.heidi.ie/ ).  The price is right, it's free. If Kim says it's good, then it's good.  Personally I would use the 6 pound hammer.