I agree with the other posts on here in that you have likely got adware or malware on your system. I would recommend you do a Google Search for Adaware and Spybot Search and Destroy, which are the two industry leaders for anti-spyware and anti adware. I won't provide links because I don't want anyone to click on a link on this that may end up also hacked and misdirecting you to a trojan site. You should run BOTH tools to clean your system and keep it maintained. This is in ADDITION to running anti-virus tools. Run both because one tool will find things the other one won't, and vice versa.
In addition, and as a start, you can install and run the Yahoo Toolbar and turn on the Spyware scanner tool built into it and run it. If you do that before you do the other two steps it may help catch and prevent even more problems.
Also, don't forget to use the Spybot Search & Destroy's "Immunize" feature. It'll help keep your system from catching known trojan programs. It's updated regularly, so keep your signature files for anti-virus, anti-spyware, and anti-adware all updated regularly to avoid problems.
Best of luck!