I don’t know, if anything OSs were quite unstable (I can’t speak for the 80s, but slightly later). When was the last kernel crash you’ve seen? We definitely have less of those now than we had in the 2000s.
What probably happens is that it assumes it is the only OS and goes into another mode if not used for a few days? (This part I’m not sure about but the previous paragraph is true). But perhaps Ubuntu is your default in the boot order? Nonetheless, try to poweroff your windows with this shift+click and check if that solves the problem.