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You probably have an unwanted toolbar. I've had good results using toolbarcleaner.com to instantly list the ones that have worked their way in, and sweep them away.
Malware scans (like AVG) don't always flag toolbars, because there are some people who actually think they are useful.
In fact, I just ran malwarebytes, and found that I had PUP.mywebsearch, with about 200 infected objects, which MWB identified.