The PSS SQL Server Engineers made a nice blogpost explaining How to Rebuild System Databases in SQL Server 2008
In SQL Server 2005, we introduced a different method than in previous versions to rebuild system databases (affectionately known as “rebuild master”). You were required to use the setup.exe program with command line switches.
This is no different in SQL Server 2008 but the command line switches have changed some and the process behind the scenes to rebuild the system databases (master, model, and msdb) is also a bit different.
The cool part about the way they are doing this is that you don’t need your DVD anymore!
You should bookmark that page immediately so that you can find it when you need and before you suffer that enormous panic attack 🙂