As long as you have the key for that computer, you can borrow or copy an install disc from someone who has one, then wipe the hard drive with something like DBAN followed by reinstalling Windows. That could take the better part of a day to do, but it's the safest.
As an alternative, you could delete the contents of your Documents folder and any other folders you have sensitive data in, then do a defrag. This is subject to your missing something and doesn't always write over 100% of everything you deleted, but it's probably good enough for the majority of folks who don't have super-secret or incriminating data on that computer.
Hello, I have an old "dinosaur" (as I like to put it) of a laptop from about 5 or 6 years ago. It's an HP, pretty big and heavy, great for a desktop alternative. I let a friend borrow it for a year or so and she said it ran super slow and was sluggish to get around the internet. She then gave it back as she bought a new one and I just put the thing in a case and in my closet.
Well, now I have another friend who wants to buy it, but I want to not only clean off all mine and my other friends stuff, but also clean it up so it's not running slowly. I do not have the Windows reinstall disc or any of the discs that originally came with the laptop, so I'm unable to just reinstall XP. What's the best way to go about this?