Ignore such messages, they are a scam.
Norton Antivirus (made by Symantec) has a very long and illustrious reputation for mangling Mac OS X systems, sometimes to the point where a complete reinstall is necessary. Among other things, it installs kernel extensions which are known to cause kernel panics and system freezes; it contains known and documented bugs which can silently corrupt Adobe Photoshop and Adobe InDesign files, destroy a user's ability to authenticate as an administrator, and (on PPC systems) can cause Classic to stop functioning; and Symantec has on at least two occasions now released flawed .dat file updates which erroneously report certain critical Mac OS X files as "viruses." (Deleting these "viruses" causes damage to the system that in some cases renders it unbootable.)
Norton has also been reported as damaging iPhone backups.
Norton Removal Tool (Symantec Uninstaller):
http://www.symantec.com/business/support/index?page=content&id=TECH103489&locale =en_US
And now this, from 11 January 2012:
Lawsuit Claims Symantec "Scareware" Warns Of Fake Threats To Sell Upgrades
http://www.forbes.com/sites/andygreenberg/2012/01/11/lawsuit-claims-symantec-sca reware-warns-of-fake-threats-to-sell-upgrades/