Your computer is most likely infected with a virus or you installed a program that also installed junk software. For example, when you download Adobe, it will install Google Chrome if you do not uncheck the box for it. They hide these boxes very easily.
I would remove any unwanted programs from you computer from the control panel and run a full system scan using:
http://www.superantispyware.com/
The free version will work perfectly. Don't forget to un-check the boxes to start a free trail! -_-;