I've edited the line to read
PHP Code
$PATH_INFO = array_get($_SERVER, 'ORIG_PATH_INFO', '');
I also disabled the "Verify that file exists" checkbox for the web site in IIS, and turned on spider-friendly URLs in the site, but when I browse to the main forums, I get the CGI error as before for the test site URL
Code
http://test.dreamlyrics.co.uk/forums/ubbthreads.php/forum_summary