Cleaning out a computer

Hi I'm not sure if this is the right forum to ask this sorry if not. I'm going to sell my sisters laptop for her on ebay because she dropped the lap top and broke the monitor. Any way my question is what would you guys suggest doing as far as cleaning up the hard drive before selling it? There isn't really anything to confidential on the computer but I also don't want anyone finding something they shouldn't. I was planning on formatting obviously but does anyone have any other suggestions? The os is xp.
Thanks

 

Sorry I should have searched harder before I asked this question. I found some good info in other threads.