depends on what you are looking for. some guy before said vista... that depends on how old the pc is! my pc for example says designed for xp and vista capable but thats just a joke i think. i get bluescreens approximately every hour. but vista is really best for games, especially for DX10. if it is a mac pc, stay on mac. Pretty nice interface but you can only work with it, nothing else(you could also run xp on a mac, the other way round it is not possible). if you are not into gaming and don't need the cool window flip effect, xp sp3 has best performace and you can be quite sure that you don't have to format your disc after 1 week. my last hint: don't go into open source... linux is just much too frustrating because you need tools for every **** and installing things is often very complicated!