A computer virus is a program that inserts itself into one or more files and performs some action. There are generally these two different phases of a virus:
Insertion phase, when it inserts itself into a file
Execution phase, when it performs some (possibly null) action
Viruses are often rewritten and adjusted so that they cannot be detected. Viruses are generally sent as e-mail attachments, for example, like a Word file. Antivirus programs must be updated continuously to look for new and modified viruses.
Microsoft strongly recommends you use antivirus software on all your computers. Antivirus software is usually installed at each of these three places: user workstations, servers, and the network.