This can be caused by soooo many different things. A couple of solutions
solve most such problems, though the one thing that helps most -- which you
didn't provide us - is a history of the problem.

So - in case my suggestions below don't solve this - please provide
information on when this began - whether it began suddenly or gradually -
whether there was ever a time that this worked correctly on your computer -
whether it happens all the time (every single session) - whether it only
happens when your browser is open - whether it happens only on some pages or
on all pages - whether you changed ISPs or ISP software or made any other
changes to your computer hardware, software, or configuration soon before
this began - and anything else that can describe the time and circumstances
of the arising of the problem.

Sometimes this can occur because the browser is damaged. A repair reinstall
of Internet Explorer solves this one.

But, as much as 90% of the time, the cause is that you have some sort of
invader on your computer - either a virus or a parasite. Run a good,
quality, up-to-date virus checker to rule this out. Then go to the Fighting
Parasites link in my signature block (http://aumha.org/a/parasite.htm) and
click on the "Quck-Fix" link. Follow out the seven steps of the Quick-Fix
protocol to check your system. Report back here what you find. Chances are
very good this will solve the problem.

I also recommend an overall "tune-up" of your computer - see the Computer
Health link in my signature and follow out the steps recommended.