Er... MSE works fine from the latest installer once grabbed. At most you need to run the installer as administrator in compatibility mode.
I don't think MSE is officially supported on WES7, but Forefront Endpoint Protection (which is the "corporate version" MSE with additional management features) is supported on WES7, POSReady 7, and Thin PC.
I have installed the MSE install.exe version 2.1. and it works fine, so i don't know why such people say that those programm might not work?!
The MSE installer worked fine since RTM - try redownloading MSE if it still isn't. As for XP Mode the services are not running - you would need to at least versionedit a hell of a lot of files to get the actual WVPC part to install as far as I could tell. Even then the stuff may be so tightly integrated into 7 but not exist in TPC.