A computer virus is a program written intentionally and without your permission or knowledge to change the way your computer operates. The term 'virus' is an acronym which means Vital Information Resources Under Siege. Most viruses are not destructive but merely display a message and multiply themselves to spread to other computers. Some viruses, however, are written specifically to damage the data on your disks, to delete files, or to erase your entire hard disk.
There are three types of viruses: a boot virus, a program virus, and a trojan horse*. A boot virus attaches itself to the boot (start-up) record of your computer. A boot virus loads itself into your computer's memory before DOS loads in, thus it takes control of your computer and infects any floppy disks you use.
A program virus infects executable program files such as .COM, .EXE, .OVL, .DRV (driver), and .SYS (device driver) files. A program virus remains dormant until you execute (run) the program it has attached itself to. When you execute the infected program, the virus loads itself into your computer's memory and usually infects all other programs you execute after that.
*Most computer experts do not consider a trojan horse a virus because it doesn't change the way your computer operates. A trojan horse is often called a "back door". It promises to be something useful or interesting (like a game), but covertly may damage or erase files on your computer while you are running them.
How do viruses spread?
A virus is dormant until you boot your computer or execute the infected program. Once the virus is active (once it has loaded itself into your computer's memory), the virus makes copies of itself and attaches the copies to other program files or disks. The virus will stay active in your computer's memory until you turn off your computer. Turning off your computer removes the virus from memory, but it does not remove the original virus from your infected disk or program.
How do I know if I have a virus?
Any time your computer operates
differently than normal, you should suspect a
virus. There are several programs you can
purchase to check for viruses. Some of the
more popular virus-checking programs are
SCAN by McAfee, NAV by Symantec, and
CPAV by Central Point. These virus-checking
programs have a list of all known viruses (this
list is called a 
virus definition) and will check
the boot record and all executable programs on
the disk against this list. You should scan your
hard disk for viruses and eliminate any that are
found. Then in the future, scan any floppy disk
before you use it.
It is also important to update the
virus definition list at least monthly so new
viruses will always be checked for.
How do I use a virus-checking
program?
Virus-checking programs are very easy to use but they do take a few extra seconds each time you use a new disk. To check a disk for a virus, execute the virus-checking program and tell it to scan the new disk. This is done differently with each virus-checking program. The instructions below will explain how to use NAV by Symantec since that is the program we use in Grand County School District.
How do I use NAV?
If your computer is on a network, refer to the reference sheet on how to add an item to your ICLAS menu. After adding NAV to your ICLAS menu, simply highlight the menu item and press <Enter>. If you are checking a floppy disk, be sure to insert the disk before pressing the <Enter> key. The ICLAS menu is configured so only one person on the network can use NAV at a time.
If your computer is not on a network,
obtain the NAV disk from your building
SysOp.
