I just completed one project in XNA, and in this project I use the line using System.Windows.Forms So now I created a new project in XNA and tried to use the same line but it compiles as an error Error 12 The type or namespace name 'Windows' does not exist in the namespace 'System' (are you missing an assembly reference?) Any idea why this would work in one project but not in the other? Thanks

You need to add a reference to the System.Windows.Forms.dll assembly to your project. Instructions here.