uhh.. this is rather bad... try to 'uninstall' it, or to remove it unless the game would not let you do a new installation.
Not sure how this have to be done in XP (perhaps a Win XP gamer could help her?) but we have a tutorial at the support pages:
http://www.protext.pwp.blueyonder.co.uk/sst-forum/techsupport.htm#AlreadyThinksInstalled
alas it's written for Win98.
good luck greetz Capt. Ed