Remember those sneaky AVG and Ask toolbars that are hidden in certain software installation wizards? There's others, but I've noticed AVG is extra invasive as it changes your default homepage and the default search engine for your address bar. The most recent time I contracted the AVG toolbar bologna there was no question. It was either my installation goes through with the AVG bar, or not at all.
These sneaky installations really benefits AVG, since they hijack your search engine option, every query sent through them gives them some money from yahoo or whichever engine they're routing to. It's funny but this is how firefox originally got most of its funding. The homepage for firefox is a google search bar, but every query sent through the firefox version gives mozilla a little coin. I recently found this nifty add on for firefox that puts all these sneaky settings back to default.
It's called SearchReset, https://addons.mozilla.org/en-US/firefox/addon/searchreset/
All this temporary add on does is change some settings in about:config