As a follow up to my answer above.
If you need to delete your invalid XP Pro before installing your valid XP Home the steps are going to be as follows:
1) Backup all your personal data off your hard disk to CD or DVD (all personal files, music, movies etc) as they are going to be lost.
2) Ensure you have all the drivers for your PC (graphics, sound etc). If you dont have these then your PC will not work properly.
3) Install your new Windows XP Home. This will delete EVERYTHING off your hard disk.
4) After the XP Home install then you need to install all the hardware drivers.
5) You will need to reinstall ALL the programs you had under XP Pro as they wll have been lost during the XP Home install.
6) Configure Windows so it is set up as you like it with e-mail, internet, passwords, favorites etc.
This is probably going to take at least a whole day, maybe more if you have never done it before.