The master database stores your logins and, most importantly, the pointers to all of your databases. Without the master database, you can’t successfully start SQL Server. To recover your master database, you need to use the Rebuild Wizard (Rebuildm.exe), located in the \Program Files\Microsoft SQL Server\80\Tools\BINN directory.
However, you may not know that RebuildM.exe utility has retired since SQL 2005. In case your system database gets corrupted, how you rebuild Master database?
You will need to use the setup.exe on the installation CD to rebuild system databases. RebuildM.exe now has been merged into the main setup program.