%drive% is a placeholder for the drive from which to Q-Dir is started.
In the case of: the Q-Dir path is "M:/Q-Dir/Q-Dir.exe" then would be %drive% a placeholder for "M:".
This allows you to define yourself,
through the menu :
Extras >> ... more options, then tab bar Association.
Q-Dir is suitable for Microsoft Windwos : Servers 2000 / 2003 / 2005 / 2008 Win 98 / XP / Vista and Windows Seven x64 and x86