Well, OTA means Over the Air, which means buying an antenna, mounting it in your attic or on your roof and pulling in whatever channels you can. The only channels you can get OTA are NBC, CBS, ABC, Fox, and other assorted local channels. Go to antennaweb.org. and put in your address and it will tell you what channels you can expect to pickup from your location.
The HDHR requires that you have cable service which it then distibutes to the devices on your network. I'm, honestly, not that familiar with it, just what I read after a quick Google search, but that's my understanding. So if you cut your cable the HDHR will become useless to you.
In order for your HTPC to utilize the OTA signal it needs a TV Tuner card installed. Hauppage makes decent cards for ~$100.