What are the best operating systems for a Home Theatre PC?

Really? Updated "October 2023" when this release hasn't been available since 2019!

MS-Windows.
Ubuntu.
Mac OS.
Fedora.
Solaris.
Free BSD.
Chrome OS.
CentOS.

No longer available:
https://en.wikipedia.org/wiki/OpenELEC

Server been updating for days now! Wonder for how long this maintenance will take place 🤔

I want to install openelec on an old Socket 939 x86 WinXP machine but I can't boot from USB drive. How can I install the software to run as a Kodi machine? Can you burn an image to DVD to boot from?

Windows 7

This is no longer supported or officially available for download.

It depends on your target platform (TV Box, computer board, ARM, Intel, etc.)

Does things like Flex run on pretty much all of these?

Comment moved from user ryanrapini:

"Windows 8.1 is believe it or not an excellent HTPC OS. My current HTPC boots into the MetroTile screen which serves as a visually pleasing HTPC main interface. I have big entirely custom tiles that I can navigate with windows remote or k400 with ease. I use the program OblyTile to make the custom tiles, icons and boot parameters, which is a MUST if you will be going this route. http://oblytile.en.softonic.com/ Sorry for the softonic link, you can find another version elsewhere if you don't want to navigate the bloatware installer.

@ryanrapini, Kodi is not an OS but a program. So this option does not really fit the question. There is a Linux based distro called openELEC that boots straight into Kodi though, so maybe this option could be changed to that?