What are some convincing arguments to upgrade from Visual Studio 6?

Question

I have a client who is still using Visual Studio 6 for building production systems. They write multi-threaded systems that use STL and run on mutli-processor machines.

Answer 1

Not supported on 64-bit systems, compatibility issues with Vista, and it was moved out of extended support by Microsoft on April 8, 2008

http://msdn.microsoft.com/en-us/vbrun/ms788708.aspx