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.

1. Open My Computer and Navigate to C:\Windows
2. If you can't see the Inf folder, you'll need to change the view options. If you can see the Inf skip to step
3. Choose Tools --> Folder Options... from the menu bar
4. Click on the View tab
5. Click the "Show hidden files and folders" radio button
6. Uncheck the box next to "Hide file extensions for known file types"
7. Click the "OK" button to apply the settings
8. Open the Inf folder
9. Right click on mdac.inf and choose "Install". You will see an install dialog come up. Let it finish. You will be prompted to reboot the server/computer.
10. Reboot

You should now see the SQL Server driver listed in the ODBC administrator. This will install any other drivers that were missing too.