Is your computer running slow? Sluggish?
Do you have to wait for the pages to load or does your homepage suddenly change to something else?
You may have unknowingly been affected by a virus or spyware.
A free online scan program is http://www.trendmicro.com Click on housecall and it will do a free scan for you. Be prepared to wait, 20 minutes up to a few hours, depending on your connection speed.
I've successfuly used housecall when my computer was sick.