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Does the screen resolution change with UAC only when launching a 3D application?
Is it the same behavior with UAC off? With different Catalyst versions?
I have never encountered this problem with my ati cards in 7. My suggestions:
1. Make sure to have your monitor driver installed, and get system updates
2. Switch to your desired resolution in windows Control Panel, and ensure that the refresh rate is set as recommended for your monitor
3. Slide the UAC control down one or two levels down from default
4. Check the Event viewer after the UAC prompt when it changes resolution