Short of restoring from a known good backup you could try:
http://technet.microsoft.com/en-us/library/ff713991(v=sql.105).aspx
even though these steps are for SQL Server 2008 and are reallly meant for application databases as opposed to system databases.
RegardsRudy KomacsarSenior Database Administrator"Ave Caesar! - Morituri te salutamus."