OK, calm down, take a deep breath, and relax.

If it's an upgrade installation, then to my knowledge you're out of luck. It's in the EULA that you can't upgrade from Windows XP MCE to Windows XP Pro, you have to buy a full coppy of XP pro. It won't let you do otherwise.
What you need to do is a clean install. Back up all of your files onto an external drive, then insert the CD and reboot. You'll boot off the CD and can proceed with setup from there. Please note that you will loose all of your settings, as well as any software installed on the computer and any documents on the drive you are installing windows onto.
However, first consider why you are doing this. There is
only one thing that Windows XP Professional has that Windows XP Media Center Edition doesn't: support for active directory. If you aren't going to be using a windows domain, then there's no need to upgrade from MCE to Pro. MCE IS pro with A.D. support removed. It still has remote desktop, encrypting file system, etc. So before you go ahead with this, consider whether you really need to. Additionally, XP MCE can be joined to a domain if you apply the
necessary changes, however, that is unsupported by microsoft.
And what do you mean by the "undo folder"??