Recreate MSDB

1. Detach the damaged msdb. You can't just detach msdb because you're not allowed to detach system databases. However, you can if you start the server with trace flag 3608. I did this by shutting down the server, navigating to the directory 'C:\Program Files\Microsoft SQL Server\MSSQL.1\MSSQL\Binn' and doing the following: start sqlservr.exe -c -T3608
2. Move or rename the damaged msdb files (msdbdata.mdf and msdblog.ldf in the 'C:\Program Files\Microsoft SQL Server\MSSQL.1\MSSQL\Data' directory)
3. Run the instmsdb.sql script in the 'C:\Program Files\Microsoft SQL Server\MSSQL.1\MSSQL\Install' directory
4. Shutdown and restart the server without the 3608 trace flag

Kaynak: MSDN