I strongly suggest that you restore MSDB from backup. You've got some nasty corruption there and repairing the system databases is not generally recommended.

Restoring MSDB isn't the same as for the user databases, check Books Online (or the MSDN website) for details on how to do that.

Edit: You should also do some investigation of the IO subsystem. Looks like some pages have been zeroed out, that's typically an IO subsystem problem.