What I did was first get a hairdryer and heat up the sticker then straight after remove it. Any residue left can be easily removed with an eraser (pencil eraser).
Might sound a bit stupid but it gets rid of the residue
I honestly don't know why people put stickers on, they look very tacky.
Could you just put the Windows 7 sticker over the top of the Vista sticker?
Didn't we just see this case in the "show us your badges" thread
I guess you're having a change of heart about the whole "look what's inside" thing
Nail varnish remover ought to do the trick for removing the glue. Any organic solvent I guess but choose wisely otherwise you'll dissolve the case instead.
I would use a damp warm sponge and soak the sticker, just don't rush it.