NT and new software

Recently removed Norton Anti-virus software and installed Symantec Anti-virus Corporate version. The system is running, however, the anti-virus is not updating. Believe that it will require a removal/reinstall to correct that, however, will it be necessary to rerun SP6A and the rollup fixes after installing the anti-virus program? Seems to be the big question, as to when to run the service packs. This system also runs Exchange Server 5.5 (SP4) Any suggestions greatly appreciated.