I was having a similar issue with my Acer laptop. The VGA port would work while booting up, but once windows started I wasn't getting a picture on my TV. I found a solution that works for me. I opened the Display properties (right-click on desktop or use Control Paned ! Display). Then I went to the "Settings" tab. It showed two monitor boxes: The first was my laptop screen. The second was grayed out. I clicked on box #2 and then checked the box under it that reads "Extend my Windows desktop onto this monitor" and pressed the Apply button. Once I did this my TV showed a signal again. Next I made sure I was clicked on box 2, and then changed the screen resolution and color depth to values that my TV accepts, once again clicking the Apply button.
Now the trick is that is doesn't mirror the primary screen, but acts as a second screen. To put something on it you slide a window off the laptop's screen in the direction of box 2. The relationship between screen 2 and screen 1 can be changed by re-arranging them on the Display tab. For example, my monitor 2 box sits to the right of my monitor 1 box in the Display panel. So on my laptop screen, I slide a window off the right-side of my laptop screen, and it slides onto my TV. I slide my mouse cursor off the right side of my laptop and it appears on my TV, where I can then work with the windows I have there.