Resolution Problem (Black borders) on Bootcamp + Win7 (x64) + iMac 27 + 4850 HD

Hi guys,
I am running Windows 7 on an iMac (i7) 27 4850.

When I change the resolution to something lower than 2550x1440 I get black borders instead of a scaled image.

This does not seem to happen in OSX, why is this?
Is there any way that I can fix it?

Hi again,
Here is a little bit of extra info about my situation, I hope this helps!!

I have tried a big amount of drivers, the ones that come with bootcamp 3.0, the ones that come with bootcamp 3.1, also the ones on ATI website for the HD4850 and also the same ones but for the mobility chip.
Same result 🙁

I read something that happened to someone which waas kind of similar. The guy said that he fixed it by deataching the external screen.
It is funny cause I did have a second screen and I removed it, I am not sure but maybe this has started happening since then, cause I clearly remember playing some game when I freshly installed the windows copy and that did not happen.
I am going to try to get my hands on that second screen again and see what happens if I connect it.

I might give it a try and reinstall windows 7 or perhaps try it out with windows XP.

Thanks again!

Funny,
so I attached the second screen and viol'a, it all works ok.
Now changing the resolution scales appropiately into the screen filling it completely.
I guess that I will just keep it there 🙁