IMO, the timeless Windows look is Win2K - it's when they got the original Win95 / WinNT4 look perfect. They adjusted the colors a little bit so it was less dark, and IIRC that was also the first version to have sky blue wallpaper background by default, and blue gradient title bars. It was also the first to switch from MS Sans Serif to Tahoma 8pt for the UI font, and Tahoma was pretty much pixel perfect at that size.
So the overall effect was that it looked more cheerful than Win9x, but not cartoonish.