My 2 cents worth:
Most computers with built-in video usually have some way to disable the internal video, be it by jumper or BIOS. If you have a newer computer at all (last 5 years) it is almost sure to be in the BIOS. If I had to guess I'd look under advanced properties or motherboard properties. I can't tell you exactly what it will be called, but if you can replace a video card, it should be easy enough to find with a little perseverance.
You can also try looking in windows device manager and see if the on-board video appears anywhere, if it does, disable and reboot. That won't help with your start-up error, but it will hopefully straighten out your windows game resolution issue.
Ok. So this may take a while. I recently tried to update the VGA of a prebuilt system by gateway and had multiple hang ips. First it has an onboard Radeon HD 3200. So i bought another VGA from the same company. It was a Gigabyte HD 7850 by Radeon as well. I thought to gat the same company to make it easier. Though throughva weeks worth of trying to get the drivers to load and failing everytime i returned it. I then sought to try an Invidia VGA to see if it would work. I bought a GTX Geforce 260 used off of Ebay. The official drivers loaded easily and the adapter is working on windows at the appropriate screen resolution which is 1440 900. When i bring the video game i play currently (swtor), it plays the game fine. The problem is while it says it is at 1440 900 the game looks stretched. However the mormal windows screen resolution looks fine. I tried toggleing the in game resolution but it is already set to 1440 900. Changing it to any smaller results in the black bar around the screen as a filler, but the game within still looks stretched left to right. So how do i go about fixing this problem. Do i need to remove catalyst completely from the computer? Or is there a different problem? How do i resolve this issue. On the onboard VGA i am able to play games albeit very clunkily and thays on the lowest setting possible. I should also say though that with the onboard Vga it has no problem setting the screen resolution to the appropriate size.
On another note there is a AMD message when i boot on saying that the AMD driver doesnt support the current graphics card but the Nvidia symbol in the bottom loads as well. So both catalyst and nvidia are loading at startup. Please let me know what i can do.