There are a couple of ways to get HDTV. You say that yours has a tuner. The most common tuner is an ATSC/QAM Tuner
. An OTA (Over the Air) tuner allows you to plug in bunny ears and capture local HDTV Broadcasts (ABC, CBS, PBS, etc, No ESPN). Clear QAM gets you HDTV channels from you cable but in almost all cases it will only get you the same channels as OTA, that is the local broadcasts. Any "cable" channels such as CNN, ESPN, etc will be encrypted.
The other less common tuner on an HDTV is a Cable Card Tuner. This requires your cable company to come in and install a Cable Card
into your HDTV and set it up, this is similar to the card used on satellite TV boxes. This will allow you to watch all of the cable channels that are available in HDTV without a cable box.
If your HDTV does not have a Cable Card Tuner then the only way to receive "cable" HDTV channels is by getting an HDTV cable box from Comcast or one from a satellite provider.
I believe that you plugged your cable directly into your HDTV, correct? If so that means your using its Clear QAM tuner and since ESPN is scrambled (encrypted QAM) it is resulting in the scrambled picture that you describe. If this is the case your either going to need to get a Cable Card for you TV (it has a Cable Card Tuner) or you're going to need to get an HDTV Cable Box. The technician should hopefully be smart enough to make sure you use the right video cable (ideally HDMI or DVI).
Hope this helps.