Often the infection will be so deeply embedded within the system that its removal may not be an easy task. Many computer viruses and Trojan viruses use special techniques that help them to disguise their presence on the computer. Not all antivirus programs have the same ability to treat established infections
At this point, the user needs to ensure that the malware is accurately removed from their system. In these circumstances, when the user finally updates the antivirus database - or installs a more effective antivirus solution - the established malware infection may then be detected on the user’s computer.
There can be a number of reasons why malicious software may have been able to infect a machine that already had antivirus software running on it.
