XP, Vista, and similar operating systems.

What a question, of course, it's Windows XP. As far as I remember, I switched to it back in the days of GTA 3. It's an excellent operating system, much more stable than its younger counterparts, and simply amazing compared to Vista. As already mentioned, Vista was completely unfinished, and patches couldn't save it, and many games didn't run on it. But most importantly, it was completely unoptimized in terms of "networking." 95% of complaints about network issues in our local network came from happy Vista owners.