Apologies for the wall of text, it looks like my formatting got eaten, I'll try again using your suggested formatting.
Graphics Card
Asus M4A88T-V evo/usb3 on board Radeon HD4250
Sapphire Radeon HD3450
AMD Catalyst Driver Version, and Driver History
Start with a fresh install, with Microsoft WDDM v1.1 drivers
Have ready the Legacy Catalys 12.6 drivers
Operating System
Windows 8 Pro x86-64
Issue Details
Steps to reproduce:
Metro Video App breakage:
Start with a fresh install
Open Metro "Videos" App
Search for Promethius 10 minute preview, stream/download and watch
Result: It works as expected.
Install Legacy Catalyst 12.6 drivers and reboot
Open the Metro Video app and try to watch the video again
Result: A black creen with spinning beads and the text "Starting..." - no video ever starts
Uninstall Legacy Catalyst 12.6 drivers (reverting to Microsoft drivers) and reboot
Open the Metro Video app and try to watch the video again
Result: Works as expected.
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HTML5 Video test.
Start with a fresh install
Open Metro IE10
Check http://camendesign.com/code/video_for_everybody/test.html , click the play button and watch Big Buck Bunny H264
Install Legacy Catalys 12.6 drivers and reboot
Revisit http://camendesign.com/code/video_for_everybody/test.html in Metro IE10 and click play
Result: Big Buck Bunny does not play.
Uninstall Catalyst drivers, reverting to Microsoft supplied drivers and reboot.
Revisit http://camendesign.com/code/video_for_everybody/test.html in Metro IE10 and click play
Result: Big Buck Bunny plays.
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Note that Desktop mode video (Windows Media Player) seems to be unaffected.
Motherboard or System Make & Model
Asus M4A88T-V evo/usb3
Power Supply
500W ATX Sumvision Power X KY300ATX
Display Device(s) and Connection(s) Used
Samsung Syncmaster 223BW (connected to HD3450 via DVI)
Phillips 150S (conencted to HD3450 via VGA)
Applications and Games
(Metro) Video
(Metro) IE10
CPU Details
AMD Athlon II X3 450
Motherboard BIOS Version
Asus M4A88T-V evo/usb3
Bios rev: 0307
System Memory Type & Amount
4GB
Additional Hardware
1x Hauppauge WinTV-HVR 67xx TV Capture card (PCI)
1x Linksys Wireless-G adapter (PCI)
Additional Details
Tested with a fresh install