Not an expert, but...If you're not using Comcast's HDTV converter box, you're not getting HD, so far as I know. But maybe that wasn't your question. I sympathize with your not wanting to pay all of Comcast's add-on prices, but I think you're getting all you're going to get, without some extra expense. As far as I can tell (have Comcast HDTV service), there's not all that much different content on the HD channels. I can think of only two, INHD and INHD2, that aren't duplicates of existing channels. And since you have ED and and not HD (set), you may not see any difference anyway. Just some thoughts.

Short answer is no, right now the only HD-DVR are either Tivo or direct from cable company. Long answer is yes you can build your own HTPC (Home theater PC) that can decode OTA (over the air) HDTV signals and record them.

As Steve_A points out you must have a box to get HDTV from your Cable Co so you might as well pay the few extra bucks and get their DVR as well. My cable co (Insight) costs $4/month additional for the DVR option.

And contrary to Steve_A I believe you will see a HUGE difference in picture quality on an EDTV set and the difference IS worth paying for IMHO. I can hardly stand to watch SD TV anymore after being spoiled by HD shows like Lost, NFL, NCAA Basketball, US Open, movies, etc.

Sony makes an HD-DVR that works with a cable card, comes with 500GB or 1TB of hard drive space, this may be something you want to look at.