Web applications and WinForms are inherently different, and should be designed differently. Using a tool to convert then cannot be recommended.
Probably you will have to move all the common business logic functionalities to a class library and reuse them while designing the UI logics from the scratch.
this is a suggestion given to me by one the web developer named scott....he is my linkedin profile friend....
If you designed the application to be multi-tier, you simply create a new presentation layer. If you didn't, it's probably worth going back and refactoring the desktop app before starting the web side.
If it's not plausible to do that, then forget about "converting" and just write the web app from scratch.