pre-installed anti-virus tools are viruses themselves!
So many novice PC users have norton/symantec installed with their PC "from new". The trouble is that such people tend not to pay the fees required to keep the anti-virus up to date. So what happens next is that these virus libraries become out of date AND...
And the worst is these novice users still think they are fully protected. They cannot/do not want to afford the prices for these products but think they are running in the background and protecting them.
Such pre-installed programs are the worst enemy of those trying to protect against viruses. They consume computer resources and are very bad protection against the latest viruses. You can easily argue it is these anti-virus programs that actually help new viruses propagate!