Not just system resources. Symantec software actually introduces security issues to your Mac, as has been proven by a number of critical vulnerability advisories over the past couple of years. If I were you I'd ditch the Symantec option and install the open-source ClamXAV for doing scans of your data files (ClamXAV won't do the system files). You won't get infected unless someone creates a Mac virus, but at least you won't pass anything nasty onto someone else in the meantime.