If you still have the install disk it came with just do a full restore opting to omit any saved or added files.
Tell the people you sell it to that they need to do updates as soon as they start using it, particularly if they are going online with it.
Otherwise you can just empty out all that you saved on it.
Likely, most of the stuff you have on there really does not matter too much anyway. If you have your ssn or any other personal info saved on it you can just do a system search for each one particularly and delete them as they appear.... start menu > search > "type in ssn etc"
You should swap out the hard drive. when you delete your stuff it really doesn't get deleted, it is formatted so it can be recorded over so to say. if you don't recored over it it, it can be read. doing a full restore might help but I would opt to remove the hard drive.
How much are you asking for it? What are the specs?
