clare.xia (2/7/2011)
..I am thinking replace the .mdf and .ldf with the corresponding files from a working MSDB. Thoughts?...
That sounds like a very bad idea. You would lose all your SQL Agent jobs and job history, backup and restore history, maintenance plans, email setup, email operators, and any DTS packages or SSIS packages stored in the MSDB database.
Do you have MSDB set to full recovery mode?