Bootcamp Windows won't display without power cable attached

Hey guys, I have a strange problem and I don't know when it first arose.

I have a MacBook Pro which I use a VGA adapter to have 2 monitors running (my laptop screen and the extra monitor). This runs fine in OSX.

When I try to use the same set up in Windows I get weird results. It works fine while I have the power cable plugged in. When I remove the power cable only the secondary monitor shows and the laptop's monitor powers down. If I don't have my VGA cable plugged in and try to run Windows with just my laptop screen, I can only get the screen to display while the power cable is in. As soon as it remove the power cable, my laptop's monitor powers off. It comes back on when I reattach the power cable. When I have the power cable in and the VGA cable in, both monitors display fine

Does anyone have any idea what this is about? I reckon its something to do with starting to use a 2nd monitor on OSX, but I have no idea where to start.

Thanks in advance!