In that case you will need to go to the directory where Calibre is installed and try again from there.
For example, if Calibre is installed at C:\Program Files\Calibre2\ , you need to type
Code:
cd C:\Program Files\Calibre2\
into the command prompt window first.
And just to ensure that the program is found type
Code:
.\calibre-debug.exe -g
Th extra characters will tell windows to look in the current directory for the file to execute.