Not only are they crap but they could make your computer more vulnerable than BEFORE you installed their expensive doucheware.
Tell me something I don't know.
My experience of Symantec/Norton and McAfee "anti"-virus products has been mostly a tale of bloated code, sapped computer resources, crashes, corruptions etc. Using Symantec or McAfee products to protect your computer is like watching FOX to get the news.
However, on top of the fact that their software is too damn big, too damn slow, uses up too damn much of a computer's hard disk space, RAM and CPU power, it has come to light and even been ADMITTED by these companies that their antivirus products "feature" vulnerabilities that could allow hackers to gain access to your computer.
What SHOULD you use? Try ignoring the hype and the cost of those expensive piles of crap instead and have a look at Grisoft's AVG Antivirus (free version here) and ESET's NOD32.
If you are used to that piece of crap Norton or McAfee you got free with your PC and have been paying to update, you will be shocked how fast and free of problems your computer will run with NOD32.