It has been my experience when I have been called in to help with problems on a Mac if I find any antivirus package installed I remove it. Almost invariably that resolves the problem. Unless of course, it has already corrupted the file system beyond repair. The primary reason is that there is no virus on a Mac. Yes, there is malware such as Trojan horses and phishing attacks. The antivirus packages can't detect or protect you from them. They are basically worthless.
The basic rules on Macs are that any third party apps that claims it will protect from virus, protect you from hackers, clean your Mac, improve performance is lying to you and should avoid.
For more about anti-virus software: http://www.thesafemac.com/mmg-antivirus/
For info about malware: http://www.thesafemac.com/mmg/