Windows. (although not sure yet)
XP at first I think. Although I have to buy a new motherboard anyway, to play Vanguard, I could buy Vista with it. With the purchase of a new motherboard its probably possible to buy an OEM version of Vista, which are way cheaper.
*rant warning*
Windows just announced that Vista release will be delayed in Europe, for legal issues. And they announced the prices this week too. Its basically changing of $ for € and than add an extra 10% :P If you take into account that we get less support and service, you can safely say we have sore bums form all the.......
*rant over*