Restoring Windows XP to factory settings on a Dell desktop?

Hi - my family's desktop computer is riddled with viruses, so need to restore it to factory settings. All data backed up. With my old laptop I did the restoring thing a couple of times - in start up mode you press F11 I believe? But that was a Sony, does that still work with Dell? I know sometimes you... show more Hi - my family's desktop computer is riddled with viruses, so need to restore it to factory settings. All data backed up.

With my old laptop I did the restoring thing a couple of times - in start up mode you press F11 I believe? But that was a Sony, does that still work with Dell?

I know sometimes you need a disc to reinstall Windows XP but am not sure if the computer came with one (it's very old), and if it did, I doubt I'd be able to find it!

Basically, any advice appreciated - the easiest way to do it please! Thanks in advance :)
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