You can restore msdb like any other database, http://msdn.microsoft.com/en-us/library/ms190749(SQL.90).aspx, if you have a backup. You can also restore master as well, http://msdn.microsoft.com/en-us/library/ms190679(SQL.90).aspx, if you have a backup.
If you do not have backups then you will have to rebuild master, http://msdn.microsoft.com/en-us/library/ms191431(SQL.90).aspx
Jack Corbett
Consultant - Straight Path Solutions
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