Where is Delphi installed? What is $(DELPHI)?

Problem/Question/Abstract:

Where is Delphi installed? What is $(DELPHI)?

Answer:

In Delphi, when specifying search paths, you can use $(delphi) to refer to Delphi's installation directory.
If your are writing an installation program, you may be interested in how to find out, where / which version of Delphi is installed.

It is in the registry at:
HKEY_LOCAL_MACHINE\SOFTWARE\Borland\Delphi\3.0
or
HKEY_LOCAL_MACHINE\SOFTWARE\Borland\Delphi\4.0