Uncheck the hide all protected system files option found in tools>folder options>view in order to see the boot.ini file at the root of the drive C. Right click on that once found and untick the read only box and then click ok. Then you can use NotePad to open it up and remove any reference in the bottom section to XP Pro which should read as "windows xp professional" all in lower case.
Once you have opened that up you should see the one for MCE you leave intact above or below the reference to Pro. Once the Pro line is removed save the corrected verion by first selecting the "all files" not default txt format. You will then be prompted about overwriting the current one where you answer yes.
You may want to make a backup copy of the current boot.ini file in case you remove the wrong item by accident. Without a reference to Pro in the boot.ini file there won't be any boot path for it in the msconfig utility.