Virus protection is the obvious need if you don't have one.
First thing I would try is deleting all the temp files that get stored. Find your Tools Menu (usually at the top), from Tools goto Internet Options, within Internet Options find Browsing History, there should be a delete option, click on the button and it will open up all your delete options. I would definitely get rid of all temporary file and cookies. Personally, I just use Delete All once in awhile but keep in mind this will delete all saved passwords on any web site you go to. This may differ just a bit depending on what version of Windows you are on. What I described above is for XP.
Try this first. It may clean up alot of junk that gets added to your computer while browsing. If it's a true virus, this probably won't help much.
There could be a million things going on but this is the easiest to try.