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Try deleting the registry entry.

Go to:
Start --> Run --> type "regedit" --> Go into the "Software" folder --> Look for the Paradox subfolders --> Look for the "Hearts of Iron" sub sub folder and delete it entirely.

WARNING: Editing the registry is a really delicate operation, do not do it if you aren't sure what to do because a mistake can have serious consequences.
 
The best advice for the future before any more modding is to always make a copy of whatever file you are going to mod first.
Keep the original in another folder.
Then you can mess/mod with the copied file and ,then if you make a mistake you can just swap the modified file for the original.