How the App Paths Registry Key Makes Windows Both Faster and Safer
Why can you start Mozilla Firefox by typing “firefox” in the Run dialog and press enter? Firefox.exe is not located in any directory in the path. The same with Outlook (type “outlook”), PowerShell (“powershell”), VMware Workstation (“vmware”) or Adobe Reader (“acrord32”). This “magic application starting thingy” works because of a little-known Windows feature based on the “App Paths” registry key.
Windows Internals