Unwanted Windows Version

Hey I can't seem to figgure this out......
One of the Volenteer Sociaties I work for has a Xtream PC and it had XP Home installed and then XP Pro was installed over it.... Or Vic Versa....

Pro is Bad but Home works fine How can I get the system to startup using the XP home system and I also want to see if I can dump the XP Pro because the version on this comp dose not belong to the sociaty... it belongs to a Former President.

Guy was trying to kill the sociaty so he could build a interpretive center in a Provincal Park...... that is a No no in Canada unless you have the communitys premission...... One hand not knowing what the other was doing.... We caught them but at the expence of the Sociaty and the Events put on by it.... No funding No Sociaty...