.NET framework is a set of program libraries required to run applications written in Visual studio .NET . Developers can use Visual studio to create application for Windows platform ,both mobile and PC . .NET compact framework is necessary for running Windows mobile softwares written in Visual studio .NET 2008 . Download .NET compact framework 3.5 .

Download.NET compact framework 3.5

Connect your Windows mobile device to PC and then run Active sync to update to  .NET compact framework 3.5 .