Why don't you try a System Restore to the point before you tried installing 2005. Since is was not completely installed, using sys restore may just remove those files.
For removing NAV 2003 they have a (removal tool) for removing all traces from your registry. I tried to search for a similar tool to remove NAV 2005, but could not find one. I only found this page, which unfortunately, is a bit more complicated.
http://service1.symantec.com/SUPPORT...c= #_Section1
If the System Restore works for you, follow the steps you previously did (to remove your old version) and I would suggest running this tool, before you begin to install NAV 2005 again. ( Rnav2003.exe removal utility) on this page.
http://service1.symantec.com/SUPPORT...=1#_Sec tion1 Hope this helps.