Is Comcast your cable television provider? Do you have a HDTV? If so, you will want to make sure to contact Comcast to let them know that you will need to the appropriate services and equipment.
Comcast doesn't require its HD viewers to purchase a separate HD cable receiver of their own choosing, instead you will just need to subscribe to their HD cable television service and Comcast will provide you with a special HD cable box. This service isn't available in every area, so even if you have a HD television, you may not be able to get HD cable if you don't live in an area in which it is offered.
Don't know if you live in an area where Comcast offers HD cable? Just check with your Comcast provider for more information or check on the website for your local provider. Comcast's HD programming offers all of your local channels, to movies and sporting events. Prices will vary depending on your location.