External Security - How to manage network security

If you use your computer to communicate with others on a network, the web or email you're at risk of infection from computer viruses. There are now more than 81,000 different viruses out there according to the McAfee AVERT Virus Information Library, and the number is rising each day. It's highly likely that you'll encounter malicious code at some point. Infection can be expensive. It is estimated that PC viruses cost businesses approximately $55 billion in damages last year.
A virus is a program designed to alter the way a computer operates without the knowledge or consent of the user. Although no virus can cause physical damage to your PC, a virus can render your computer useless.

Are You At Risk?
How do you know if you've been infected? Here are some of the common warning signs:

" Your system slows down suddenly
" An unexpected message appears
" Files or data are lost, corrupted or unexpectedly changed
" Some files or data are unexpectedly sent via email
" You see unexpected activity on your computer
" Programs function incorrectly or not at all

Email viruses are common. You'll receive a message that contains an attachment; when you double click on the attachment, the virus code starts running. The code sends copies of itself to everyone in your Outlook or Outlook Express address book. It might then start corrupting files on your computer. Viruses aren't the only examples of malicious code that you will come across. The most common threat comes from worms programs with the ability to copy themselves from one computer to another. Worms normally propagate over the internet, and can expand from a single copy incredibly quickly. The MyDoom worm generated between 50,000 and 60,000 new copies per hour. Most email 'viruses' are actually worms; they don't damage files on your computer, though they may fill your disk and memory with spurious copies of themselves. The third member of this ugly tribe is the Trojan horse. Trojans appear to be harmless but are malicious and can do a lot of damage. Some common uses of Trojan horse programs include rounding (carving off small parts of payments from a large number of accounts or transactions), making illicit payments to their host and stealing security information.