Hi Nikolay,
I have to investigate this further, but there should be no problems installing PixInsight on an arbitrary folder, in theory. With C:\PCL or C:\PCL64 it should work fine, but let me make more tests.
Anyway, the correct way of installing Windows applications is on the appropriate system folders: either "Program Files" or "Program Files (x86)" on 64-bit systems. The old C:\PCL behavior was incorrect and compromising compatibility of PixInsight with future versions of Windows.
Happy to know it's working for you now.