While there are servers with dozens of processors theoretically capable of running M$ Datacenter Server (or whatever the name of that incarnation of NT) - see, for example, this monster from Unisys - it is far more difficult to find real-world example of 10000+ users transactional app actually deployed on Windows than on some Unix, OS/400, zOS, and they tend to be more expensive, Evil Empire FUD notwithstanding.

To make your Web app behave like desktop version (field-level validation, quick selection popups, dynamic menus, fast scrolling of large data sets...) neither portals nor .NET per se will help. You will have to use something client-side, making your thin client gain some weight. You can force-feed it JavaScript (perhaps communicating with back-end using XML a'la SAP NetWeaver), Java applets (which are just easily deployed desktop apps in different guise), something from MacroMedia... In general, it will be difficult. Take a look at goodies in Apache Jakarta project, in particular Struts. They are not magic bullet, but they halp. The price is quite enormour amount of semi-manual coding in Java, JavaScript, HTML and XML.

Alternatively, you can try to redesign your app so that it is reasonably comfortable despite being 'typical WEB' - that is, mostly screen-level validations, somewhat slower inter-screen transitions etc. The upside is that you will make the app with 10% of time and effort, if you use some of the rapid application development tools. My company uses Panther in conjuction with either BEA Tuxedo or IBM WebSphere (or MS COM/DCOM/COM+ for smaller deployments). Note that you can still mix-and-match JavaScript, Java or ActiveX components.

Good luck!