We are in the same boat, in that I have a lot of old monitors I'm trying to keep useful. But it's an uphill battle.
First, I need to make a clarification, lest somebody rips you off: There is no such thing as an HD antenna, although you see them advertised. The broadcasts here in the US use the same UHF frequencies (and some VHF frequencies) that the old analog TVs used, and the antennas are the same. I get over 100 channels over-the-air with the same antenna I've had for 15 years.
It sounds like only your projector is HD, and it is likely 720p. So it looks like you need to stay with standard definition on most of your stuff. For those older TVs, a cheap OTA converter box may be the easiest solution. I've found them at swap-meets and on eBay for around $10. If you get one, make sure it comes with a remote. You would need to run coax-cable to each converter-box, so you might also need a distribution amplifier for your antenna.
You could also find media-players that output both component and composite video (Boxee doesn't, but most others do). S-video is not very common, although my old Mediagates
, which I got on eBay a few years ago for around $30, had S-video as well as component and composite. For my 1080p TVs, I use a Dune Smart D1
(expensive) and a Netgear NeoTV-550
(cheap), which both have component video as well as HDMI.
My current project is to run my antenna into an HDHomerun
network-attached dual-tuner, and that would make the over-the-air channels available anywhere on my network (I have the whole place wired for Ethernet). I don't have the software working to my satisfaction yet.
I don't do much net-surfing on the TVs, You-Tube doesn't work anymore since their last software update, and my NetFlix is still DVD and BR based. But I can say that some online content will not work well on standard definition TVs. An inexpensive Roku box would probably work best for that, but I can't be of much help there.