as though you've got a dialer installed somewhere, as the others have said. This will have installed via an ActiveX script, which came from a web site you've visited at some point.
The phone company will not usually allow you to deduct the cost of these calls from your bill, as of course they only have your word for it that you didn't dial them yourself, although it's well worth a try.
Apart from following GANDALF <|:-)>'s advice I suggest that you go to tools in Internet explorer and select Internet options. Then click the security tab and make sure the slider is set to 'Medium', This will stop any unsigned ActiveX scripts from running.