System database can never be detached.
There are specific ways to restore backups of system databases, and it's different for the different system databases. Consult books online for full details.
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)SQL In The Wild
: Discussions on DB performance with occasional diversions into recoverability
We walk in the dark places no others will enter
We stand on the bridge and no one may pass