The Microsoft .NET Framework is a software framework that can be installed on computers running Microsoft Windows operating systems.
Many programs are written for .Net, and to use such a program the .Net Framework must be installed. It is important to use the correct version of .Net Framework.
There is no problem having different versions installed on your computer.
The C-Pen Core software requires .Net Framework Version 3.5. It can be installed directly from Microsoft: http://msdn.microsoft.com/netframework/cc378097.aspx