Just to strongly reiterate what DG has already said, if you have any third party toolbars on your browsers, delete them! They're a bit like having a permanent open door straight into your computer. The convenience they might offer is really not worth the potential problems they can cause.
When you're installing programs, such as Yahoo Messenger for example, it's always better to select the custom install option, because if the program has a toolbar option, the standard install usually has that selected automatically. So if you select custom install, you should have the option of not downloading the toolbar.
