I recently came across a Windows Server 2003 box that was missing the SQL Server driver from the ODBC Administrator. After some research and trial & error I came across a really simple way to fully reinstall MDAC 2.8 SP1 on Windows. You can't simply download MDAC from the Microsoft site and install it since its part of the OS.

You will need your Windows CD (or at least access to the i386 directory) to complete this process.