Go to Eschenmoser at the Stauffacher stop and walk to the top floor.
They sell a 200W US -> Swiss transformer that can handle the
XBox 360 (which uses about 160 Watts). I think it was about 60 Francs
or something. Unfortunately they don't sell any beefier transformers
(those you have to order from electronics stores in the states), but the 200W one is perfect for an X-Box.