Getting the guide data for the missing channels can be tricky. In my case, I had to tell MC that I am connected to the Cable TV company's digital package. Then I was able to map the sub-channels to the appropriate channels in the cable company's lineup.
Then I had to remove the channels that I do not subscribe to. In my case that was a LOT of channels. but it got me the guid data that I needed. I think this has changed in Vista and is now easier, but I do not remember settign that up.
1: Dimension 9200 - Vista Ult, 2.4GHz Q6600 Core2Quad, 4GB RAM, 160GB, 750GB, 500GB USB, Nvidia 8300GS, PVR-150, HDTV Wonder, Vbox 3560, X360, DMA2100
2: HP m376n - MCE 2005, 2.6 P4 w/HT, 1GB RAM, 250GB, 400GB USB HD, Nvidia 6200, Emuzed MAUI, X360