This means that you need to purchase the game again if you will be playing on PC even if you already purchased the game on Xbox.

Modern Warfare 2 Multiplayer Start-Time\n\n Xbox, PlayStation and PC in the United States will all go live at 9pm PT on the 27th, which translates to midnight ET on the 28th. You can see the map below for what that translates to east of the US. But the game will go live on all platforms at: 9pm PT (27th)