You've installed new software recently, haven't you!
When installing or downloading new software, ALWAYS check the 'custom' installation beforehand to ensure you are only downloading what you need. If in doubt - don't install.
Too much of this stuff comes bundled with toolbars, search engines, homepages, etc. Especially if you choose the 'recommended', 'express' or 'normal' download options without checking the options.
Even then, if you uninstall the original program, there is no certainty that you will rid yourself of the problem bundle that was included.
Hence your problem.
BUT - remember Google is your friend.
Somewhere in the 'netsphere, someone else has already asked the question.
You just can't stop fiddling, can you!