Viruses can be classified using multiple criteria: origin, techniques, types of files they infect, where they hide, the kind of damage they cause, the type of operating system or platform they attack etc.

A single virus, if it is particularly complex, may come under several different categories. And as new viruses emerge, it may sometimes be necessary to redefine categories or, very occasionally, create new categories.

When a virus infects your computer, it may have the following effects:

• The infected file may make copies of itself and take up all the free space on your hard disk.
• A copy of the infected file may be sent to all the contacts in your e-mail contact list.
• The virus may wipe clean your hard disk drive, deleting your files and programs.
• The virus may install hidden programs.
• The virus may open up a "back door" to your computer, allowing intruders to remotely access your computer or network.

ALWAYS run an up-to-date anti-virus program and use it's real-time protection.

Up-To-Date means that you update the program weekly to make sure the virus definition files are current. You can set up most software to check for updates automatically. Check your software and make sure the virus definitions are current at least once a week. Real-time protection means that your anti-virus program checks all incoming and outgoing files for viruses and stops the transfer of files if it finds one.