I'm currently using Win7 version, but I would prefer an older version.
I don't use any of the new features, and they've changed some things around that, uh, probably only I would notice. I don't use any of the new brushes and the anti-alias stuff would make it impossible to do a lot of the stuff I do. I made a quick "sketch" while high one night and fucked it up because i didn't realize I forgot to change it off the anti-alias brush. I can see why people like the new stuff, and some of them are pretty cool, it's just not for me.
Though the line tool, now, is much better. It's also quicker to access the "rotate/flip selection" options.
And speaking of modern Paint, I inspired someone on another forum to open Paint and screw around. He used the new stuff and made a very cool Bob-Rossy type scene