Hello there, Moolro. Welcome to SevenForums.
Have you disabled the Onboard GFX Card in the BIOS instead of disabling it via device manager? that could be the problem.
Try to do this:
1) uninstall the video driver
(if you have one installed)
2) boot up and go into the BIOS, disable the onboard graphics (can't tell you exactly where is that option)
3) keep the monitor connected to the HD4650, boot into windows
4) install updated drivers
5) chill back and enjoy some nice coffee.
Here's what you need: AMD Graphics Drivers & Software Download Details - Microsoft Download Center - DirectX End-User Runtime