Windows 7 Home Theatre PC

I've finally got my silent Windows 7 HTPC behaving. It had sat idle for a good while after I thought it couldn't both play 1080p video (stuttering indicating insufficient CPU horsepower) or multi-channel audio (crappy onboard ATI 4250 HDMI). As this took me the best part of 4 hours to research, fix and tune I thought I'd ( document this to save others the hassle )