Well, you should always accept your antivirus software's updates (that is, if you know that the updates are from them) but it is conceivable that they caused this. It happens, unfortunately. If that is the case, they will most likely push out a fix before long.
As for toolbars and such: remember that many of these companies offer a toolbar of some sort when you install for the first time. Doesn't mean you need it, only that they struck a deal with Yahoo or someone else.
In the meantime, try installing Firefox if you don't already have it. See if the problem goes away. Then you'll know that it's an IE/Windows problem.