can't export to Excel from SSRS but can export to CSV???

What could the problem be:

• http://social.msdn.microsoft.com/forums/en-US/sqlreportingservices/thread/bb49d7be-e9fa-4439-b46c-5ead51a6feeb/
• http://social.msdn.microsoft.com/Forums/en-US/sqlreportingservices/thread/43a6ae97-bafb-4911-b9f2-10aaf8907d9d/
• http://social.msdn.microsoft.com/Forums/en-US/sqlreportingservices/thread/733c79d0-e534-4b1c-a21f-c0b0a403a872/