Computer viruses have many different categories such as malware, worm, spyware and Trojan. All computers get viruses including Mac's, anti-virus programs really do not do anything to prevent a computer from getting a virus. They will not get rid of a virus once a computer is infected, they are only intended to prevent a computer from getting infected.
The reason viruses can not be removed is once they are in the computer they do damage that is irreversible, if an anti-virus program says it has gotten rid of the virus there is still damage to the operating system that has to be repaired. Anti-virus programs do not repair computers, only a technician can repair the damage from an infection. The owner of the computer maybe able to reinstall or rebuild the operating system in order to get rid of the virus, however, they may loss their data or if they back-up and save their data they will only be backing up the virus and reinfecting the computer.
When someone feels like their computer has a virus it probably does, when having this feeling it is best not to do anything with the computer that involves using personal information. The best thing to do is disconnect it from the internet and do not use it until it is fixed.
What makes my computer repair service different from other services is I am able to reconfigure operating systems and repair the damage that has occurred from a virus. I can back up peoples personal data and clean it from embedded viruses before transferring it back to the repaired computer.
There are many different reasons some people do not want their computers reformatted and I understand this, I also understand reinstalling the OS is usually the fastest and cheapest way for dealing with viruses. Ether way I can get rid of any virus from any computer and fix the damage guaranteed, I also install the best anti-virus software protection after the viruses have been removed.