If your looking for accuracy there is a 3rd party Website
http://www.virusbtn.com that regularly tests all the virus software manufacturers.
Symantec are one of the ONLY to score 100% on all tests. Many other manufacturers fail these in the wild tests and known threat tests.
I know Symantec software can be a resource hog at times though a lot of this has been addressed in the 2007 release.
BTW Larry which virus was it? It is rare for one of our scanners not to pick a virus up, also what version were you running?