At DreamHack Summer 2007 there was a survey from Symantec to see how many PCs from gamers were infected by malware or viruses. The results are stunning:
The results have now been published and it seems that as many as up to 80% of all gamers have been infected by viruses, malware or experienced hacking or phishing attacks. The reason is simply that they turn off their firewalls, antivirus and anti-spyware applications to improve performance, and then forget to turn them back on. The problem isn't that they turn them off to gain performance, there is a minimal risk of being infected when gaming, but that so many forget to turn them back on.
The basic problem is that both firewalls and antivirus applications have tendencies to be real processor and memory hogs, and the Norton suite certainly has a reputation of being one of the worst.