My Geforce 7800GT is not detecting my Sharp HDTV...

Ok so I have a Sharp LC-26SH12U & and a EVGA GeForce 7800GT that both work perfectly fine as far as I know. I have my GFX card hooked up to my PC monitor and everything working good haven't had any issues.

The other day I was trying to use my Sharp HDTV as a PC monitor through the VGA cable so I can watch movies on it, so I hooked up my VGA cable and everything. I can see my PC boot up on the HDTV and Windows XP starts loading but after that the screen goes blank and my HDTV says there is no signal.

Having no clue to what the problem was I can see that my GFX card was not detecting my HDTV so I braught my brothers Toshiba Sattelite laptop which had Windows Vista on it and my HDTV worked on it completely fine with it.

So I'm just wondering how can I get this to work with my Geforce 7800GT. I tried changing the refresh rate and still no luck. Am I missing something?