Aaaaaauggghhhh.... as bcastner runs from the room pulling his hair out.
I know from the newsgroups that many are less than happy with NAV 2004. The only sane way to test this is to uninstall, or use MSCONFIG to disable, the NAC stuff from starting.
I will warn you that a regular Control Panel, Add/Remove Programs, does not always cleanly remove Diskeeper.
I believe there are some manual uninstall instructions on the Executive Software site for Diskeeper.