I didn't find any other specs about your computer than the crossfired video cards, and even those don't match in your first and last posts in this thread. Are they of HD3800 or HD6800 series? Or am I just missing an explaining line somewhere?
Anyway, the problem may just be the lack of video memory! You see, crossfiring doesn't add up the total amount of dedicated video memory, or at least that's what I've read. Since most of the 3800 series models only had 512 MB which is the recommended minimum, you might be hitting the limits especially with the advanced graphics maxxed. The HD6800 series have 1024 MB which will allow a little more headroom for textures, but even that isn't too much. Also, if your entire computer is from the HD3800 era, it's starting to have performance related issues with max eye candy.
The first third of your DxDiag would help to see whether it's your hardware or something else. That would also help to determine whether you would benefit from upgrading to 7/8 or not.
As for Avast, you might want to add AcesHigh into Settings->Antivirus->Excluded.