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?