To install the MGA driver, copy rxddi.dll to the %windir%\system
directory and replace the existing %windir%\system\mga.drv with
the 3DDDI-enabled mga.drv file in this directory.

If you are currently running the MGA driver, you will need to
reboot to DOS to overwrite the old mga.drv.  You can get to DOS
by using the "Restart the computer in (M)SDOS mode" option in
the Shutdown dialog within the Start button.