If you need to know if client's browser runs on a Vista machine, run javascript
navigator.userAgent
or
navigator.appVersion
and search the returning string for "Windows NT 6.0". If it is there, you are on Vista.
Soon you won't need to go to the server to treat your client-site javascript with some c# calls. Watch this technology, it looks promising:
Silverlight.
Scott Guthrie:
Silverlight opens up a ton of opportunities to build significantly richer client experiences that run cross platform and cross browser.
For .NET developers, it means that you can now write .NET code using any development language (VB, C#, Javascript, Python, Ruby and more) in the web-browser (using Silverlight), web-server (using ASP.NET), in desktop applications, and with mobile devices. You'll be able to use great Visual Studio developer tools and Expression Studio designer tools to target each of these experiences.