The company I work for is creating a Toolbar that you can install into IE. We going to be supporting most versions of Windows and IE. Has anyone ever tested a toolbar? Do they have any general information about it? Am I going to have to test every browser/os combination?
It would have been great had you told, what does that tool bar do...but based on information you provided, i would suggest the following as per my experience-
1. As you told, thet ur software suppports a lot of OS and browser, it is imperative that you should test run on all the available browser-OS matrix. Create a matrix of Browser-OS on side and test to be executed on them at other. It will help you to estimate the effort that is going into testing these.
2. As far as functional areas goes- lay stress on testing the installation/uninstallation of toolbar. As it is going to be integrated with IE, it can impose problems and is certainly a high risk area.
3. Test exclusively for each and every functionality your toolbar offers. Break the whole functionality into simpler functional areas.
You need to create metrix of what all the OS you are going to test then go for the Browser versions you are going to test. Make sure you run the different kind of applications like Java/asp etc., to see how the toolbar behaves.