It has been my experience that Browser Helper Objects (BHO) cause the most problems on your computer. The problems are caused by “spyware” that hooks itself into Internet Explorer and then opens pop-ups or slows down your computer whenever you run Internet Explorer.
To check what BHOs you might have installed, you can either use HijackThis! or BHODemon. These both show you a list of all BHOs installed, and then you can choose to remove or “unhook” them. Even if you don’t have any spyware installed, it may be a good idea to run the software now so you know how to use it in case you end up with a problem.