Maybe in the Seinfeld world Jerry received a copy of Windows 95 as a gift from someone (like the Drake or something), and that someone was obviously unaware that he had a Mac (of course, who would know that anyway, since he never uses it). Jerry, being the kind of guy he is, wouldn't have the heart to tell the guy, "Hey, look, I'm sorry, but I really don't have any use for Windows 95," so he just keeps it on his desk.
Maybe.
He *ought* to have a copy of OS/2 Warp on his desk...