:IsRegValue Key Value -- succeeds if Key and Value exists
::                    -- Key    [in]  - registry key
::                    -- Value  [in]  - registry value, return default value if omitted
SETLOCAL
if "%~2"=="" (set v=/ve) ELSE set v=/v "%~2"
reg query "%~1" %v%>NUL 2>NUL
EXIT /b