The only thing I've ever used is Windows. Is there something simpler and more easy to use for someone who knows nothing about the intricacies?
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If you don't know, I would ~not~ do it. I have never had a reformat that ~didn't~ go very very bad. This is something techies do... but is akin to telling a non-mechanic to take your engine apart just to clean everything.
You could accidently turn it from a working computer into a boat anchor.
I would uninstall everything that he won't need, delete files of yours, and then do a de-fragment on it... That is easier to walk you through.
I'd love to tell you that you should upgrade it to Windows XP, but I would look at the system requirements first to see if it has the power to run it.