A computer virus can cause unusual screen displays or messages, but most don't do that. A virus may slow the operation of the computer, but many times that doesn't happen. Another common indication of a virus infection is a change to interrupt vectors or the reassignment of system resources. Unaccounted use of memory or a reduction in the amount normally shown for the system may be significant. A computer virus is a program designed to spread itself by first infecting executable files or the system areas of hard and floppy disks and then making copies of itself.
Viruses usually operate without the knowledge or desire of the computer user.
To activate a virus or Trojan horse program, your computer has to execute some type of code. This could be a program attached to an e-mail, a Word document you downloaded from the Internet or something received on a floppy disk. If your e-mail or news software has the ability to automatically execute JavaScript, Word macros or other executable code contained in or attached to a message, it is strongly recommended that you disable this feature. If an executable file shows up unexpectedly attached to an e-mail, you should delete it unless you can positively verify what it is, who it came from and why it was sent to you. The recent outbreak of the Melissa virus was a vivid demonstration of the need to be extremely careful when you receive e-mail with attached files or documents.