Which Unit Test framework should I start with for C# in Visual Studio 2008? (Windows Forms app)

QUESTION - If I'm starting a Windows Forms application in C# using Visual Studio 2008, which unit testing framework should I use?

There seems to be one build into VS2008? Or should I look into something like NUnit? Or is NUnit what is used in VS2008 under the bonnet? What's the most popular.

Would be good to have something that allows for: (a) mocking/stubs, and (b) ability to assert exceptions

thanks