I have problem with Visual Studio 2012 (OS: Windows 8.1 Preview)
In my case, updating Windows Phone SDK and VS Express 2012 helped on Windows 8.1, but performance is poor. If it's helpful try installing update to VS:
http://blogs.windows.com/windows_phone/b/wpdev/archive/2013/07/02/running-the-windows-phone-emulator-on-windows-8-1-preview.aspx
And updating SDK:
http://dev.windowsphone.com/en-us/downloadsdk