Chrome OS

Any idea how to get it on an old windows tablet? Possible or not worth the headache?

You can look up your particular tablet model on google for available roms, etc. It depends on the tablet - if it is windows RT tab (sometimes restrictions), or full windows tablet with bootloader unlocked (allows to flash roms). Really, it may not be worth the hassle. For tablets anyways, android roms may be a better choice than chromeOS