This answer on [how to restore a mac to a factory fresh appearance][1] will do the trick for you. Secure erasing your old user folder or using Disk Utility to wipe all the free space would be a good precaution if you want to be sure the data is overwritten (barring an SSD being installed). 


  [1]: https://apple.stackexchange.com/questions/12996/how-to-install-apps-to-snow-leopard-after-a-fresh-reinstall-and-still-keep-the-ma/13012#13012