I don't have any problems with a Swedish version of XP SP3. Some filters don't work. Think it is my crappy programing...not sure. Do you have some device/wall between that computer and ettercap? I have bumped into some that are "smart" and detect poisoning.
Can you one way poison? You using ettercap 0.7.3 on BT4 beta? Whats your command line look like?