you may have a dialer which unknown to you can redirect your computer to a premuim number without your knowledge.
unfortunatly you will be responsable for payment of this bill.
download spybot search and destroy,update it and run it.remove everything it finds.
physicaly disconnect the computer from the phone line once you download spybot.
run it often to keep your system clean and its a good idea to maybe ask BT to put a block on premium rate and international numbers for your line.