Anti-virus programs protect your PC from viruses, worms, and other malicious programs. Without an anti-virus program installed, your PC is vulnerable to infection.
Installing an anti-virus program isn't enough. It must be enabled to work. Until the anti-virus program is enabled, your PC can still be infected by computer viruses.
An anti-virus program cannot protect a PC from the very latest viruses, worms, and Trojan horses unless its list of viruses is kept up-to-date. Until the newest virus definitions are downloaded and installed, your PC is vulnerable to infection.
When you have multiple anti-virus programs on your PC, they can interfere with each other and may result in a situation where your PC is being unprotected. For maximum protection, you should run only one anti-virus program at a time.
If you don't regularly scan your computer for viruses, new viruses can infect your system without you knowing it. To protect your PC from infection, your anti-virus program should be run regularly and frequently — at least once a week.