If you have a CORRUPTED EXE.XML in the %PROGRAMDATA% folder, caused by a bugged installation of another product (we had reports that UT Live beta cause this), this will prevent ALL executables from being loaded. It's a bug in P3D V4 which should be fixed in the next update, and LM is aware of it.
Remove all your additions from the XML files, and be sure they are not corrupted. An easy way to check if an XML file is corrupted, is to open it with Internet Explorer.