If your key is a flick out type it appears to be a reasonably common fault - what has happened is that the rubber buttons have stretched and are permenantly in contact with the microswitches underneath, this activates the global open/close system which includes the windows (hold down the unlock button and the windows open, hold down the lock button and they close)

There are two solutions one expensive and sure to work - visit your dealer and get a new key (Timpsons may be able to help cheaper) the cheaper solution is to open the key as though you are replacing the battery and remove the circuit board - you'll have to break the little plastic retainers that have been heat formed on the rods that hold the board in position but a drop of Bostik or similar will replace these on reassembly. Using a blunt cocktail stick press the rubber through the hole where the switches were to streach it back away from the microswitch. Reassemble and try. Good Luck.