Same with my Directv HD DVR, the shows look exactly the same as watching HD TV. Is it possible your DVR is not HD, it should say on the DVR itself if it's HD or not, Comcast provides both SD and HD DVRs. A brand and model# will clear up that part of the issue.
Also you might run a search on the AVS forum about Comcast, seems that they have a lot of problems with their boxes as well as their HD programming.

A computer once beat me at chess, but it was no match for me at kick boxing.