If you have another msdb backup from different server with same build can be restored but you loose all the history or jobs etc...

http://blogs.msdn.com/psssql/archive/2008/08/29/how-to-rebuild-system-databases-in-sql-server-2008.aspx

http://sqlblog.com/blogs/tibor_karaszi/archive/2008/08/27/rebuilding-msdb-on-sql-server-2008.aspx