My limited experience with some (arguably, frail) HDMI to coax products makes me wonder about your PC. Do you have VGA output? If so, that would be an easier solution, at least for the video signal.
We can help if you provide more details on the capabilities of your PC (make/model would be good).
I have a zinwell ATSC converter box because I use a pair of rabbit ears as an antenna to a really old analog TV. The RCA cables go from box to TV. I have one RF input coax slot left on the box.
In this set up, is there a HDMI to coax cable that can be run from a PC to the converter box so I can run video from the computer through to the TV? Specifically, I want to be able run video clips in either .AVI, MP4 or MOV through to be played on the TV. I don't even know if this is possible.
Other option: can I run the videos from a PC to the TV if there's a cable that goes from HDMI to a 3 head RCA?
The only input to the TV is RCA and coax.