have you tried using the other HDMI input, or using different HDMI or HDMI-DVI cables? The only other thing I can think of is to connect the pc straight to the tv, and change the resolution to 640x480 then reconnect to the receiver and see if you get a picture then, if so, try readjusting the pc's resolution back to 1080p. I'm guessing since it works when connected directly to the tv, that it's not an HDCP issue, so maybe there is a handshake issue between the 3 devices.
UPDATE:
I have the PC into both HDMI Ins on the 663 and it still not working (both Ins work on the Cable box).
I think your are right in that it's not an HDCP issue but a handshake problem in that even when changing the resolution back, it still doesn't work through the receiver.
I called Yamaha support. All I got from them was that "there is nothing in the receiver that should cause a problem for the HDMI signal". I said could it be a compatibility issue between the video card and the receiver? He said it could be. I asked if there was any record of issues between the 663 and my card and he said he didn't know because they don't keep records of problems between video cards and the receiver b/c those are the video card manufacturers problems not yamahas. I said either way they become the consumer's problem and it would be helpful if there was a list compiled of known issues.
Now I have to decide if I want to:
1) Replace HTPC with Bluray player (sucks that I bought some HD DVDS for a a great price b/c this LG player in the PC could play both)
2) Change our video card in computer (seems like a crap shoot)
3) Change receiver (there goes the budget)
4) Purchase Blu-ray and HD DVD player (then I guess I would have to get an HDMI switch as the 663 only has 2 HDMI ins and I am also running HD cable box).
No good answer I suppose. Anyone have other suggestions?