No antivirus, no matter what brand, can possibly be 100% effective 100% of the time. All antivirus companies charge for professional virus removal, but there are always other things to try, see the last link in my signature below.. OK that will take longer but at least it's free. Basically, with the paid service, you are paying for the convenience of having someone deal with the problem for you in an expeditious manner.
Any reputable malware removal forum will tell you basically the same.
AntiVirus Products are directly dependent on the health of the Operating system wherein Installed. A corrupt OS will not only compromise Security but also adds more load to the AV product that is Already Protecting PC from Threats.
Secondly any Any AntiVirus is Totally Controllable. What if you accidentally trusted a file which might be a virus. An AV works with all the Previleges that you give; you can give the Product full Permissions or restrict it from making changes.
One more Scenario where you get attacked by a virus that its new to the Internet. It will take time for all AV companies to update to their Threat Database and provide an update to detect / delete these New Viruses.