sounds like spyware/adware got through. Try Adaware or similar program to
get rid of it. I went through this recently. It is very frustrating and
can be tedious to completely eradicate it from your system. You often need
to go through your registry and hack out lot's of crap, remove files
including cookies and when you think you have it removed, you don't.
check this out for more info: http://www.spywareonline.org/index.html