I just want the game engine to run at a constant 60 Frames Per Second on any computer.If all you care about is 60FPS, then call Present() with the D3DPRESENT_INTERVAL_ONE flag. This will enable vsync, limiting your FPS to the monitor refresh rate.
This probably isn't the most reliable method if he intends on getting 60fps across the board. The number of 120hz monitors are increasing rapidly, so those individuals will be getting locked at twice the speed of others with 60hz displays.