Why do you need to connect the cablebox to the PC TV tuner? Get a PC TV tuner that can decrypt cable, and you won't need the complexity of setting up IR blasters so that PC can change channels on the cable box, and you will not be limited to SD only content. You will get full blown HD (1080i) the that cablebox would have output through HDMI.
Why complicate things?
6 TV's in the house on FiOS and we only pay $4.99/month to connect them all!!! Power to the CableCard and WMC7!!!