Erasing personal data from macbook, while keeping some applications?

Hey All,

So I might be in a bit of a pickle - I'm selling my macbook and need to erase all my data. I've pretty much googled that part out and know how to go into disk utility and erase free space, as well as deleting my user. But I realized two things: when I sold it I advertised that it had microsoft office and snow leopard (I upgraded that from regular leopard but don't have the disk).

SO, my question is: Is it at all possible to securely erase my personal data but keep it from reverting to its original operating system and possibly keeping the office programs?

Thanks!