Reboot your cable/dsl/whatever router- many of them cache dns and if it's messed up or getting too big, that could cause this.
If that doesn't do it, consider apps you've downloaded recently: some of them "optionally" try to change your default internet search. An old/clumsy attempt e.g. not allowing for UAC so changes are left incomplete, could cause a similar problem.
"... They ne'er cared for us
yet: suffer us to famish, and their store-houses
crammed with grain; make edicts for usury, to
support usurers; repeal daily any wholesome act
established against the rich, and provide more
piercing statutes daily, to chain up and restrain
the poor. If the wars eat us not up, they will; and
there's all the love they bear us."
-- Shakespeare: Coriolanus, Act 1, scene 1
