Because it doesn't support type 2 pixel shading.
I upgraded to a 9550 and everything works fine.
Supported ATI/AMD Cards:
a.. Radeon® 9500, 9550, 9600, 9650, 9700, 9800, X300, X600, X700, X800, X850 series
a.. Radeon® X1300, X1600, X1800, X1900 series
a.. MobilityT Radeon® 9550, 9600, 9700, 9800, X300, X600, X700, X800 series
a.. MobilityT Radeon® X1300, X1400, X1600, X1800 series
a.. CrossFireT Xpress 3200 series
a.. Radeon® Xpress 200, 200M series
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Peter
Toronto, Canada
XP Pro SP2 x 2 + Vista RC2 5744
P4 HT @ 3.0ghz, 700gb HDD, 2.0gb DDR
Creative SB Audigy 4 Sound, ATI Radeon 9550 Graphics
TS H429A + H552U CD/DVD Combi-Drives
Sony DRU820A x 2 DVD-RAM Combi-Drives
Why this graphic card does not support Vista Aero?
Wojtek