i'm guessing microsoft are threatening you with all kinds of nastiness because you downloaded a pirate upgrade.... that's the only reason i can see someone would want to downgrade from the fairly nice Pro to the comparitively poor Home.
answer is: I don't know. You can probably uninstall the pro upgrade somehow, unless MS never thought to provide it, because under normal circumstances no-one in their right mind would want to do it. Have a look in add/remove programs in the control panel, including the Windows Components section.
Other than that, you might be stuffed. If you can't find these options, your best path may be to get all your files off there, reformat the hard drive and start again from the original XP Home installation disc that (....hopefully) came with your PC. Do check first though - the recovery files might be stored somewhere on the HD instead, e.g. in a second or even a hidden partition, and you may need to use a special program to access them.
I don't suppose the installation of this dodgy Pro upgrade created a system restore point when it loaded, did it? you could even use that, if so. maybe.