Hi Kathy, welcome to the forums.
As Eddy said you should remove NAV first, but you must use the windows add remove programs interface (for NAV) first before running RejZor's AEC tool.
RejZor's AEC, NAV removal tool only gets rid of what is left behind after NAV's uninstall program. If you use this first and then try to use the windows add remove that may fail.
Also make sure that you select the correct version of NAV 2003, 2004 when using RejZor's AEC tool.