ASP.NET fails to detect IE10 causing _doPostBack is undefined JavaScript error
Try installing the Browser Definition Files from NuGet http://nuget.org/packages/App_BrowsersUpdate into your project. Browser Definition files for ASP.NET 4 are compatible with ASP.NET 3.5, so you should be okay. You may have had an issue installing the hotfix you mentioned if you do not have the appropriate permissions on your machine.
if you continue to have this issue, there is a small code snippet you can add to your global.asax that will resolve the problem as a temporary workaround until you have your server(s) patched.
The KB 2600100 hotfix were replaced by Microsoft .NET Framework 4 Reliability Update 2 (KB2600217) http://support.microsoft.com/kb/2600217 - the documentation states that this update resolves 7 issues including two that you mentioned. But it seems that you need to use Microsoft .NET framework 4 instead of 3.5. If the KB 2600100 hotfix is not working, you might want to check IIS Server Manager to find out what .NET framework version is being used for running your web site.