Running Second Life in a window has some advantages.

- You will be never be set to away while working with other programs

If you have Second Life maximized, you can work with other programs in front of Second Life.

Each time you move the mouse over “the background” the avatar is not away anymore.

- You can switch between applications much much faster.

I have a quad core cpu/2GB/vista/nv 8800 and does not see any differences in performance, quite the opposite, i sense that the game is even faster in window mode. The frame rate might be a little bit slower, but the whole game seems to be faster.

You can change the option edit..->preferences.. check or uncheck the option “Run Second Life in a window”.