xp install stuck

hello, I would like some help with an install failure please. I have an older system that needed a reinstall of xp sp2, and as it was still going but hanging, I tried a repair first,( original authorised xp disk) but... when i entered the serial code, it was not valid. tried several times, no go.(Ihave a suspicion that it has been swapped as i had to ask for it back once from a mate and he took a while to return it! anyway, I cant get it back now and I'm stuck in the install/repair with windows wanting the key but no way to get out of this screen, (i tried restart, safe mode exit etc, but always goes back to install). Now I don't care about any of this, all i want is to get out of this loop so I can reformat and start again with ubuntu or something, but i'm stuck with no imput. there must be a way, hope you can help.
cheers

 

this has been resolved, by using the original image disk, but curious to see no body had an answer for this problem, surly it must occur