If it's all the time then it's probably related to PlayReady/drm not being set up properly. Did you run the Digital Cable Advisor after you installed the InfiniTV?
Okay, so after completely rerouting all of my home theater's cables, I was able to look into this some more. I know things weren't very smooth when I set it up the other day, so I wanted to take a look.
I recall running into an issue where I tried to run something, and WMC stated that there was an error executing the program or something like that. Well, I believe that "something" was the Digital Cable Advisor. However, WMC immediately swapped to what appeared to be the tuner setup. So, I figured that was the DCA, and I ran that.
Tonight, I downloaded, installed, and ran the Digital Cable Advisor, and it said that my tuner card was setup. I cancelled its next task, but then ran it again, and allowed it to continue on figuring that it wouldn't hurt. This still didn't enable HBO. Then I opened the HBO On Demand channel (I assumed it wouldn't work), and it told me that the PlayReady update was incomplete. I allowed it to install, and it still doesn't work.
One thing I want to do is call up Comcast to ensure that something isn't weird with my account. My apartment comes with basic cable + HBO, and I don't know if anything weird happened to my settings when they added the HD package. I would check online, but they won't let you do that for corporate packages.
EDIT:
I opened the Ceton diagnostic tool and told it to tune to 301 (HBO HD), and here's the result:
http://i.imgur.com/uhu83.png
It sounds like that's not a PlayReady issue if the card says it can't decrypt, right? PlayReady just deals with displaying the video... I think.