This sounds like a driver issue to me.
Open Device Manager and check the status of 'Display Adapters'.
You can get there by opening: Control Panel/System/Device Manager
I all is OK there (ie, no little yellow warning triangles) then it is odd.
But I am still leaning towards a faulty/corrupt driver installation.
ATI has an official Driver Removal Tool on their website; it is supposed to completely remove any remnants of their drivers from your system.
I would use this tool, then do a clean install of the latest drivers available for your graphics card.
This may or may not help, but it can't hurt, either.
If you continue to have problems, post back here and I'm sure someone smarter than me can help you.
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