I use Vista x64 and the OS installed the drivers for my T500 automatically, and it just worked in WMC without needing the bundled software. I have however given up on WMC and installed the 3rd party TV app since, because I found MC to be very user unfriendly. It detects channel numbers incorrectly (radio 4 is on 804, whereas every other TV app, TV and DVR tunes it correctly on 704) It totally fails to detect or tune the "red button" channels so I couldn't use it to flip to "Final Score" to find out how Pompey's match was going, and it has no interface to edit the order of channels, so I was unable to set it the way I have all TVs etc set up with all the BBC channels together, followed by all the ITV etc etc. It's also very clunky in use (switch from small window to fullscreen in any other app, and the picture just gets bigger instantly, but MC actually stops the transmission and restarts it with about a 1 sec gap between)
It also lags behind the transmitted signal by about a second more than any other app.
But if you really want to use it in MC, I'd start by downloading the latest x64 drivers from the Pinnacle website, and updating them.
What message do you get from MC ? Did it tell you it cannot detect a tuner like mine did with the AIW before I put the T500 in ?