my vote for Windows XP
it is the system I use since 2002 ideal for internet retrogames emulators,new games old games and everything works there
it's today the most compatible OS which you have drivers for anything
also installed vista mixer and an unoficial patch to use DIrectX10
sincerely I do not use Win 7 because I found retro-compatibiliy problems with certain games and software
for example my TV card not works fine on Win 7....my prefered joystick is game port and there are not gameport on winvista or win 7
