I developed a small winforms application for myself in Microsoft Visual Studio 2008 Professional Edition at my workplace, the Visual Studio is licensed to the firm I work at.
If I want to sell that application, what are my license options?
EDIT:
The issue here is not my relationship with my employer (the code was written after hours, we have an understanding) but my relationship with Microsoft.
Ex. if I continue developing in Visual Studio Express can I keep my old code? Is there a way to verify if some assemblies were written using a Visual Studio Professional?
I'm having trouble seeing what the issue here is. Assuming your employer has a properly licensed copy of Visual Studio, they have the right to let you use it for any purpose they choose. It's no business of Microsoft's whether or not they let you use company resources for your own project.