Virus
A program or piece of code that "infects"
one or more other programs by embedding a copy of itself in them. When the virus is executed it spreads the "infection". This normally happens invisibly to the user.
Anti-Virus Software
Most anti-virus software will detect programs that may allow remote access to your computer (Trojans), send unwanted e-mail to those in your e-mail program's address book through your e-mail address (e-mail worms), or perform activities or functions that may corrupt data on your computer. If you decide to use an anti-virus program, remember to keep it updated so you will be protected from new viruses. Here are links to just a few of the many anti-virus programs out there:
Further, good virus protection doesn't end with anti-virus software. To prevent infection of your PC, you must take a few common sense precautions. For example, NEVER open an e-mail attachment from someone you don't know. Even if you do know the person or organization from which the e-mail appears to come, you still need to exercise caution. That person's or organization's computer could be infected with a virus that e-mailed itself to you.
If you receive an e-mail warning of a dangerous virus that's spreading fast and advising you to pass on the information to all your friends and co-workers, look before you leap! Many such warnings are hoaxes that can flood servers and cause congestion on the Internet. Again, the anti-virus software sites are often your best bet for ferreting out the hoaxes from the real threats. Links to a few of their hoax listings/databases are listed below for your convenience: