When upgrading my notebook to Windows 8 I encountered a strange behavior on opening an ASP.NET Web Application hosted on a Windows Server 2003. The dropdowns of the ASP.NET Menu Control didn’t work anymore.
First, I thought this must be a problem of IE10, but this was only partly true. The problem is that the .browser files on the Web Server do not reflect the current versions. You could manually update the ie.browser file in \Windows\Microsoft.NET\Framework<version>\Config\Browser or use the fixes proposed by Scott Hanselman: http://www.hanselman.com/blog/BugAndFixASPNETFailsToDetectIE10CausingDoPostBackIsUndefinedJavaScriptErrorOrMaintainFF5ScrollbarPosition.aspx
After applying the (machine-wide) fix everything should work normally.
Markus