1) Add a VB.Net class libary project to your solution.
2) Add the Crypto.vb file to the class library project(Newly created VB.Net Project).
3) Add a reference in your Windows Forms application (C# Project) to your vb.net class library project (VB.Net Project).
4) Use the Crypto class in your Windows Forms application (C# Project).
To summarize the action -
Currently you are having solution with single C# project in it.
After change you will have two projects in your solution. One with VB.net and the other in C#.