Basically, there is no tool to convert a Windows Forms app to an ASP.NET app. The ways each work with it's own forms are so different, a direct translation is not possible. You're stuck re-writing your application. But, as Colin said, if your application was written properly, meaning that you kept your business and data logic in classes seperate from your application code, you've minimized the work you have to do rewriting your UI code. Properly written Business Logic and Data Access layers will be completely independant of the User Interface code. This way, you only need to write a new UI layer if you want to support multiple types of clients, like Windows Forms, ASP.NET, Pocket PC's, mini browsers (cell phones), or Embedded systems. Google for "3-Tier" and "n-tier" application development and you'll find TONS of resources on how to properly write applications that support this.
Dave Kreskowiak Microsoft MVP - Visual Basic