This is a new install on a new computer. Win 10 Home Alienware X51 R3 i7 3.4 Ghz, 8gb ram GTX970 4gb. I got to he launch menu, set up everything and click play. No error messages, even got the pop ups from Windows game mode. I have the Fallout 3 logo on my taskbar and can hear the music fine, but that's it. I try alt-tabbing to the game but nothing happened, other than occasionally getting a mesage "error an instance of Fallout 3 is already running". Task manager showed Fallout 3 but no error.

Did you remove the drivers for the other graphics card? Did you install the optional updates for Windows?