I’ve been asked to integrate a windows form application with SagePay to take payments directly from the application. The SagePay documentation talks about ASP.NET so I’m not sur
You can post to urls and get back a response via .net objects and parse the results to display on a form.
You can also ask SagePay to create a Vendor account with 3d secure switched off. This allows you to take payments in house in your winforms app without the customers password however most card vendors if you bypass 3d secure will not protect you if someone calls and uses a card fraudulently.
However it can definitely be done.
You can normally post to secure servers from non secure pages so again this isn't usually an issue.