Freely sharing computer programs and system disks. Downloading executable software. If your computer begins to do things out of the ordinary, or when it stops being able to do things it has always done in the past, it may have been infected by a virus. It is important, though, to remember that problems which come from corrupted system files can look very similar to virus symptoms. Try to rule out more standard causes before assuming a virus.
Prevention is a matter of vigilance and avoiding contact with unknown disks.
Most anti-virus packages contain routines that can perform each kind of task. Back up your files. Viruses are one more very good reason to back up your files. This will prevent the virus from spreading to your original disks. Obtain public-domain software from reputable sources.