NAV Setup Error

I have uninstalled NAV v5 because auto-protect would not enable. Trying to reinstall NAV, the following error occurs:
‘UnInstaller setup failed to initialise. You may not be able to uninstall this product’.
Click OK to this error, and it takes youback to the desktop. NAV will not install. I have tried removing all reference to NAV in the registry but still the error occurs. This is happening on Windows NT 4 with SP3 (updating SP is not really an option). I have tried different copies of NAV 5 on CD. Any ideas would be gratefully accepted.