Windows 7 "not genuine"

I bought a compaq presario c700 from someone and it has windows 7 on it but it says its not genuine when i tried to run windows update to see if I could get it to be genuine it tells me it comes with vista preinstalled so I take it the person installed windows 7 themselves how can I fix this I dont have a disc for either porgram but I do have a xp pro disc will it be compatible or does anyone have any other suggestions.