if your saying you pluged another PC with your new HD GFx card installed in there, and your same HDTV worked fine then it cant be the TV can it!
or did you mean you pluged it into another totally seperate HDMI capable HDTV you have downstairs ? in which case it may indeed be the first hdTV, to be sure id take that TV downstairs/or PC upstairs and try it on the known working PC and HDMI cable first.
it sounds like a bad setting in your first failed PCs OS needs sorting to me.
i have the same problem right now with getting a composit connected 2nd display activated and working.
its an OS registry problem, i may need to reinstall the OS from scratch to fix it if and when i make the time to try....
but ill probably jst get a DVI to component lead and DVI to HDMI long enough sometime before that to try first as im in no hurry right now.
for a quick test so as not have to reinstall an MS OS to confirm it really works.
if you have the time and the understanding you could always try and find a linux livecd with the
http://search.virginmedia.com/result...0+X2+driver&cr=
installed and ready to use with that HDMI cable and let us know which one simply boots and works out the box for you if any....