As I said you have two choices really:
You can switch your Desktop colours to 16 Bit and made the game run Windowed mode.
You can try Oemga Drivers or go into the advanced control panel of ATI and try and few different settings.
The Only problem I ever had with ATI Cards is the black Border around the pop up boxs (yours are invisible) when I am in town - I dont feel the need to Alt Tab out of the game and even if I do, it seems to work fine.
If you google ATI & Mir3, you will see many people have a similar problem - did you get the LATEST Drivers for your card? Are you sure you are using ATI drivers and not WIndows Update ones?