Virus: A computer program that replicates on computer systems by incorporating itself into shared programs. Viruses range from harmless pranks that merely display an annoying message to programs that can destroy files or disable a computer altogether. Whether they're considered malicious or malevolent, all viruses spread rapidly. For example, from one computer to millions of others around the world, infecting machines and causing them to crash. Some well-known examples include the "I Love You" virus, code red, and NIMDA.
Viruses are most commonly transmitted through e-mail; "strains" have appeared that use personal e-mail address books to propagate themselves from machine to machine. If you are connected to the Internet or any other network, it is important that you take precautions against viruses. Get a virus-scanning program (in fact, you can get one from this definition on NetLingo.com) and do not open any e-mail attachments from people you do not know.
Virus Lists and Libraries:
http://www.viruslist.com/eng/
http://virusall.com/downrem.html
http://www.cexx.org/
http://us.mcafee.com/default.asp
P.S. The information provided has been a WPW community effort. Thank you for the donation of time and information everyone who has contributed and everyone who will :)