My Dads PC has been acting a real arse for a while, he recently bought an external HDD and a few other things. He wants to put his XP back to factory settings to start fresh, but I understand Microsoft removed the option to restore back to factory settings in the "System Restore"

He doesn't have the original XP disk as it was pre-installed by [whisper]PC World[/whisper] but he has been upgraded so it isn't a PC World Product anymore

But how would he put it back to factory settings without the XP Install disk?

AFAIK, it's perfectly legal to use somebody elses install disk, or even a downloaded image, providing you install it using a valid licence key (which should be stuck to the side of the case, or something).

Maybe press F11 or is it F12 as soon as you boot the pc up, that works on most pc's, don't know if it works on them all but all the computers I have had in the last few years have had the feature.