When you uninstall drivers you have to completely clean them out before you install another one, because anything left over from the previous install will conflict with the other.
To uninstall graphic drivers completely, first download a program called driver sweeper. This will clean out the remains of the graphic driver after uninstalling it. Here's the link:
Guru3D - Driver Sweeper (Setup) download from Guru3D.com
Install that but don't use it yet. First you have to uninstall the ATI Display drivers from add/remove programs.
After that reboot into safe mode, and then use driver sweeper. Just select ATI display, analyse, and then clean. Then it will ask you to reboot.
Reboot and install the latest drivers.
If that doesn't work, you should test the extra graphic cards and monitors that you do have.