I'm using this combo on three different machines, no issues with NAV or LiveUpdate that I'm aware of.
Check Symantec's Knowledge Base; if you have the error message handy, search on it. I've had very good luck in the past with these types of issues. There's also a forum that's provided me with a wealth of info...
Maybe something got mucked with during the SP4 upgrade, have you tried reinstalling NAV or doing a repair?