you may have a virus or some other nasty on your system like a dialer.
do you have any antivirus software installed?
if so update it and run a full system scan.remove anything it finds.
if you dont have any AV software get this free one click here dont forget to update it after install,then run a full scan.
next get spybot search and destroy and adaware click here click here both free also.these will remove spyware and dialers etc.update them after install and run a scan with both and remove anything they find.
if you are still having problems post back.