ok, so you plug monitor in and it works 100%

you then plug the TFT in and the PC still boots etc. but no display on the screen?

not even when its POSTing etc.?

sounds to me like your GFX card is outputting a signal that the monitor cant display (most common when you try and set a refresh rate of say 70 on a 60hz tft, it just shows no signal

altho HOW you can do that before it even posts (as it should defalt to 60hz on startup) i dont know