Hi Tonyme, it's a driver to do with network communication.
However...
Symantec has released a new version of the Norton Removal Tool that handles the 2008 versions of their product, and presumably addresses these network driver issues.
After running it against my failed NAV 2008 installation, I was again able to install NAV 2007 on Vista 32. The new 2008 Symantec network monitoring drivers appeared to be properly removed (can't be done manually).