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 ?

You are going to have to invest an awful lot of time into making a web site look like your existing Windows application. If you have to also make it work on both major browsers, even if you can limit yourself to just the latest versions of both then you can easily add another 50% of effort to make it look/behave the same on both IE and Netscape.

If you in fact only need to make the application work over the Internet, then why not just use web services on the webserver and have your Windows application call the web services to get it's data.

This of course doesn't gain you any cross platform portability as a web approach in theory would give you. However in practice if you are trying to produce a rich client feel with a website you will have fun even dealing with the differences between IE on the PC and IE on the Mac, let alone the differences between IE and Netscape.

If you re-wrote your Windows app using .NET then all the web services stuff comes for free. Even if you stick with the exist app it should be too bad. Assuming your existing app is using ADO you can insert an extra layer between the bit that consumes ADO recordsets and the actually data access. You can then have you new web services layer build ADO recordsets from your web services call and pass them up to the layer above. Then you would need less changes in your existing code.

I guess if you re-wrote the app in Java you would get some platform portability and you could still take the web services appraoch.