If its happening in other things as well i would suggest running cpu-z and seeing what speed the CPU is running at when running the video etc. CPU usage shouldnt really be enough to require the cpu to step up its speed though.
I have read the thread but the only 2 other suggestions i have is find the latest audio drivers for your system and update it as that can cause problems, also to download process explorer and look what if anything is using more CPU time than it should be. If you can see the DPC which is near the top hitting something like 10/15% often then its generally a driver/hardware issue. All you can do in that case is do what you have done and update firmware and drivers. It could be the ATI card you have has a firmware update available, i know some of the 4800 series cards had issues at one point.
As its not happening in XP it does suggest more of a driver issue though.
I have read the thread but the only 2 other suggestions i have is find the latest audio drivers for your system and update it as that can cause problems, also to download process explorer and look what if anything is using more CPU time than it should be. If you can see the DPC which is near the top hitting something like 10/15% often then its generally a driver/hardware issue. All you can do in that case is do what you have done and update firmware and drivers. It could be the ATI card you have has a firmware update available, i know some of the 4800 series cards had issues at one point.
As its not happening in XP it does suggest more of a driver issue though.