I am an experienced WinForms/WPF/Silverlight developer and have an interview for a product that is ASP.Net. I am very familiar with C# and .NET, as well as standard HTML and CSS (from rails programming).
So what else should a WinForms developer learn before and ASP.Net interview?
If you're doing ASP.NET WebForms, you should read up on ViewState, the ASP.NET page life cycle, differences between user controls and server controls, JavaScript, the AJAX Control Toolkit (not my favorite controls, but a lot of ASP.NET projects use them) and jQuery (comes with VS 2010 now).
Edit: ViewState vs. ControlState or just State Management in general, http://msdn.microsoft.com/en-us/library/75x4ha6s.aspx. Also, an obvious one I omitted that MusiGenesis mentioned, MasterPages.
If you're going to be working with ASP.NET MVC, then check out haacked.com and the ASP.NET MVC site. Can anyone else suggest other good ASP.NET MVC links (I've barely used it so not a pro on this topic)?
I think that's a good starting point.
You must have a copy of Visual Studio, so make an actual ASP.NET application before your interview. Use a master page, attach a data grid view to a real database, create a user control and use it on two different pages, create a page that collects a few pieces of information (text, a date, a number etc.) and submits it, and code that saves the info to a database. Add a javascript-based client-side tool of some sort (a date picker or FCK editor or something like that) and work it into the overall application. Finally, add and integrate some kind of reports - PDF is always good.
If I were conducting the interview, and you had done the above things successfully (even just once) prior to the interview, you would be able to answer most of my ASP.NET-related questions, and that - combined with your overall experience - would probably get you the job (assuming you also bathe regularly).
A good interviewer should recognize the skills you have in the areas you've worked on and not care if you don't have the specific skills for this particular job. If you can demonstrate that you know the c# language and BCL and WinForms really well, the fact that you learned that well is a good indication that you'll also learn ASP.NET well when presented with an ASP.NET project.
Very very important is to understand the way things work in ASP.NET applications:
The top three specific ASP.NET WebForms topics I encountered in interviews a couple of years ago were
These days an awareness of ASP.NET AJAX controls such as the UpdatePanel would be useful and an appreciation of ASP.NET MVC and jQuery are increasingly valuable.
However, since you don't have professional experience of ASP.NET you might do better to emphasise your general approach to professional programming, knowledge of design patterns etc. Also consider how you might answer questions around why you're interested in an ASP.NET role when your background is in WinForms.