Like hRex said, reformatting your hard drive will keep the average Joe from ever being able to access any of your files. However, if the average Joe is very curious about what you had on your hard drive, he can hire a not so average Joe who may be able to learn more than you want him to be able to learn. I guess deciding how secure you need to be depends on what you have had on this computer and what you think about the people you are selling the computer to.

Also, like he said, you will have to re-install Windows and re-install all the programs that came with the computer.

If you are serious about getting rid of anything that was ever on the computer before you sell it, I would look into Killdisk . The free version is probably good enough for getting rid of anything that even most professionals could find. If you want the version that is supposedly up to department of defense standards for erasing data, you could get the version that costs money. I think that would probably be overkill unless you have secrets that are important to national security...