Andy pointed me at a feature of Expression Web that does a similar job to the Balsamiq user interface designer:
I had a quick play and you can make diagrams and design the navigation between the forms in your use interface. However, I found it a bit more complex to use than Balsamiq (although it has to be said that it does an awful lot more, and actually lets you construct the interfaces as well as play with them).
For students the interesting thing is that it is available for free from DreamSpark:
If you are a student on an academic course (and have an Athens username to prove it) then you can get hold of all kinds of goodies here, including operating systems and Visual Studio. You can even get XNA Creators Club memberships so you can deploy your XNA programs to an Xbox 360. Well worth a look.