I have this client still rocking XenDesktop 4. For reasons too boring to explain here, we have yet to upgrade yet. We have been adding some physical machines to the XenDesktop mix and I had to install the XenDesktop agent on a physical Windows 7 machine. Double Click the MSI for the VDA and you will get this error :
[The version of Citrix Virtual Desktop Agent x64 is not compatible with Windows Display Driver Model (WDDM) display drivers.]
The quick fix to install the VDA on the WDDM Windows 7 machine is to use the following command line.
<Location of XDSAgent.MSI>msiexec /I XDSAGENT.MSI INSTALLONWDDM=1
This is actually the first half of the HDX installation.
Once the VDA is installed, you can proceed with the HDX extensions on the physical PC (referred to as the host in most of the documentation).
Of course, even after getting the components installed, Windows 7 on Physical machines is no picnic.