Does the new version of the application actually need to be a web application, or does it just need to be accessible over the Internet?

Actually we could just use web services, but recently we've been burned by a bug getting out and the client not wanting to release a patch to fix it (too many users to send a patch out to even though the bug is severe - you really don't want to get me started on this situation though). This has bolstered the idea of being able to roll out a web version because it would be easier to fix when problems are encountered.

You are going to have to invest an awful lot of time into making a web site look like your existing Windows application.

I was under the mistaken idea that .Net would give us this ability easily. I see now that this was an incorrect assumption. So I believe that look and feel is going to have to take a back seat to functionality (imagine that!), but as long as I can come up with something that isn't too confusing to the users there shouldn't be a problem. We'll just have to make sure there are some user-benefits to the web version to make sure they all move over.