There are tools for this, but then again the current generation of IDEs like Visual Studio are just as fast to mock up a design as would be a graphics tool.
If you insist on a non-IDE software, try Microsoft Visio. It comes with some built in stencils for the "core" windows controls (a little dated with traditional menus, toolbars, etc.. no ribbon UI or even Vista UI) but you can also download more stencils online and create your own stencils for frequently used pieces of work. One area Visio excels at is the ability to add annotations, notes, arrows and symbols to your drawing.
Many developers take a hybrid approach by taking a screenshot of your UI from your development application and pasting it into Visio. Now you can decorate the screenshot with all the little annotations and explanations while you discuss it. (This is also a good technique for documenting your UI).