The most likely cause of this is if a file has been deleted or corrupted. Does repairing the SQL Server management tools fix the problem?
Repairing SQL Server is a command-line operation. You can repair by inserting the installation media into the disk drive and typing the following:
start /wait <CD or DVD Drive>\setup.exe /qb REINSTALL=SQL_Tools90 REINSTALLMODE=OMUS
You can find more information on installing SQL Server from the command line here: http://msdn2.microsoft.com/en-us/library/ms144259.aspx