Firstly, you will have to make sure you have removed everything you added from where you added it.
Secondly, you will have to delete the cache that was created (which the game will be loading each time so the error is still happening)
You can delete the cache folder by browsing to C

Documents and Settings\username\Application Data\Sports Interactive\Football Manager 2008
If that still doesn't work you should try doing a clean reinstall. There are instructions for that here (although it's for FM 2007, but the same still applies)
http://www.sigames.com/sibase.php?type=view&id=209