By default under Windows, calibre will install into C:\Program Files\Calibre2 unless you install the 32 bit version on a 64 bit system in which case if will be installed in C:\Program Files (x86)\Calibre2. The other exception is if you have installed the portable version of calibre in which case you got to decide where it was installed.
So "C:\Program Files\Calibre2"\calibre-debug.exe -g is likely what you would need to use to run calibre from the command prompt. If that doesn't work, try "C:\Program Files (x86)\Calibre2"\calibre-debug.exe -g. If you copy and paste, include the "s since the path has spaces in it. The version below uses a variable for the location of the Program Files directories just in case you are using a non-standard location. If you are using calibre portable, it's left as an exercise for the student to locate the executable.
Code:
"%PROGRAMFILES%"\Calibre2\calibre-debug.exe -g
"%PROGRAMFILES%(x86)"\Calibre2\calibre-debug.exe -g