There are so many different ways to mock up prototypes of application screens and web pages. From the good old favourite pencil and paper then scan it to an image all the way to the hard core designers favourite illustrator.
I'd probably have to say that just HTML in either Expressions or Visual Studio would be the main one for me as it's agile, quick, easy to change and is already a start into the development process.
However if you don't know HTML and probably don't want to there are still a stack of options for you. These include basic Microsoft PowerPoint, which surprisingly isn't that bad and has a lot of effects you can apply to fields and button to a very cool FireFox application called "Pencil". If you are looking for a cheap quick and easy way to prototype an application interface I'd say Pencil is definitely worth a try.
If you use Pencil I'd love to hear some feedback or if you have a better alternative let me know