So it looks like Aero is somehow sucking the power out of the GPU and leaving less processing power to do the deinterlacing? Did you ever try disabling aero and setting vertical refresh to always on as this seems to fix the the tearing issue for me in XP?I can't say why. With Aero on you'll get only Bob & Weave for H264-acceleration on Vista x64. Without Aero it works. But without Aero Mediaportal does not run well (tearing...). In the German forums some other guy had the same experience. He bought a Radeon 4650 now and that works well it seems.
And also: The Radeon 4550 still gives you a flickering gui, the 4650 not (I asked him).
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