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Have you installed any form of p2p file sharing software lately? Some of the better known outfits install "scum ware" along with the file sharing software.
If not try running ad aware this might help you to locate the problem. You might also want to take a look within your ad - remove programs options, it is possible you may have installed something that came with a download. You might be able to remove it from there. If you are sure the results are ah-ha they also might be interested in this. Drop them an email explaining your situation.
Hope this helps.