Intenta lo siguiente:

1. Uninstal NF.
2. Delete all files of NF and enfuns (enfuns is in program files).
3. Delete registry with CCleaner.
4. Open run (for xp) or type in search box (for vista/7) temp and delete files.
5. Do the same with folders prefetch, %temp%.
6. Open run (for xp) or type in search box (for vista/7) regedit.
7. Doble click on  HKEY_LOCAL_MASHINE -> doble click on SOFTWARE and find folder/s SD EnterNET and delete them.
8. Then when you are sure that everything from NF is gone instal the game and play :)