I don't know if anyone else noticed, but the flying dumpster in the Firefly episode "Trash" was running Windows XP. I wonder if that was a subtle jab.
I'm pretty sure it's XP in Classic mode, at least. It looks like the second window from the front is the Printers folder in Tiles view. The wizard in front tells me that it's either 2000, ME, or XP, and 2000 didn't have Tiles view.