If your computers have suddenly started to run slowly, your computers keep crashing or freezing up, or browsing the internet is slow, the likelihood is you have a computer virus.
A Bit About Viruses
Computer viruses are the "common cold" of modern technology. They can spread swiftly across open networks such as the Internet, causing billions of pounds worth of damage in a short amount of time. Five years ago, the chance you'd receive a virus over a 12-month period was about 1 in 1000; today, your chances have dropped to about 1 in 10.
Viruses enter your system via e-mail, downloads, infected floppy disks, or (occasionally) hacking.
By definition, a virus must be able to self-replicate (make copies of itself) to spread.
Thousands of viruses exist, but few are found "in the wild" (roaming, unchecked, across networks) because most known viruses are laboratory-made, never released variations of common "wild" viruses.
Virus behavior can range from annoying to destructive, but even relatively benign viruses tend to be destructive due to bugs introduced by sloppy programming.
A virus is just a computer program. Like any other program, it contains instructions that tell your computer what to do. But unlike an application, a virus usually tells your computer to do something you don't want it to do, and it can usually spread itself to other files on your computer -- and other people's computers.
If you're lucky, a virus will execute only a benign "personality quirk," such as causing your computer to make seemingly random bleeps. But a virus can be very destructive; it could format your hard drive, overwrite your hard drive boot sector, or delete files and render your machine inoperable.