Originally Posted by rbinck
It would be helpful to know what receiver you have. I'm assuming it has more than one HDMI input and you have connected both the PC and cable box to different inputs.
Your problem could be the PC is not set for a HDTV resolution. I would start by connecting the PC to the TV directly and booting up the PC to see if you can get a picture. Most of the time during boot a PC will ask the monitor (in your case a HDTV) what resolution is best and will set up the video card accordingly. If that does not work then I would get a computer monitor attached to the PC and set the resolution to a HDTV resolution. Since your TV only has a single HDMI input, I'd assume it is an older HDTV (it would be helpful to know the brand and model #)and some of those are very picky about what signals they will accept. If you can get the PC video directly connected then you should be able to reconnect through the receiver.
My receiver is a Sony STR-DH510. I have the Cable box and pc connected to two different hdmi ports on the receiver and i believe the receiver has 3 hdmi in and 1 out. I have had the pc connected to my tv directly for a few years now via hdmi and it has worked perfect. The tv is a 22" samsung (cant find model number searching online but will be able to get that later when im home. It is approximately 5 years old. 10,000:1 contrast ratio, 1080 I.When i connect the cable box via hdmi to the receiver and to the tv from the hdmi out on the receiver i get perfect audio from the system and perfect video from the tv.