My Main systems are on either Windows 10 or some flavour of relatively recent Linux, But I've also got systems running Windows 7, Windows XP, and Windows 95. I also have a PowerMac G5 running OS X 10.4, and a PowerMac G4 and iMac G3 running Mac OS 9.2.2. Technically I have Windows 98 Installed on an old laptop but it doesn't POST anymore. so I'd hardly call it "running"...
The Windows 95 system is too underpowered for the web today. the iMac G3 is just on the cusp- using Classilla I can use more "basic" sites (like ComputerHope) without much fuss. The PowerMac G4 and G5 are held back more by the browsers not having newer features than the systems themselves not being capable of running those features. But Both can be used reasonably competently- the PowerMac G4 can play 480p youtube videos without a problem for example, so it still has some use left!
I use Windows XP on a Pentium 4 for some older Windows games which are easier to get running on it. It also gives a nice separation, so I don't have a lot of old software installed on my main PC. (older games like to change fullscreen resolutions and stuff which causes some annoyances on my Main system)
I also have Windows XP on a Thinkpad T41. I bought the system for a few dollars as I like Thinkpads. It actually came in handy. Recently I got a lot of 12 broken Nintendo gamecubes (I like buying broken stuff for cheap and trying to fix it) one of them had some kind of rather rare Modification which allows it to run homebrew software and has a bunch of interesting tools, but the driver software only works on 32-bit Windows, so I just moved the laptop (they are portable, after all!) and I was updating the firmware and backing up memory card save files and such in a jiff.
