That isn't 100% accurate. You can get the OS from information that the
browser provides through ASP.NET or through JavaScript.
The ASP.NET provided method is to use the Request.Browser.Platform property.
I just tested it again, and it displays WinXP for my Windows XP system.
The JavaScript method parses either the navigator.userAgent or sometimes the
navigator.appVersion although you will have to do your own translation such
as Windows NT 5.0 is actually Windows 2000.
The first time I looked into the things that JavaScript can access, I was
pretty suprised. Some searching on Google can answer all of your JavaScript
detection questions.
Ian Suttle
http://www.IanSuttle.com