Virus and Security Information
Virus Scanners

Viruses are malicious code attached to otherwise innocent files that spread automatically, and can cause damage to your system and data. Anti-virus software protects your computer by checking incoming files and existing files on your hard drive for known viruses, and cleaning or deleting those files. Note that the best way to protect yourself against viruses is to be careful about how you use the internet. Never trust files that are sent to you by email, or which try to automatically install themselves, especially if the file names end in .exe, .com, .vbs, or .scr. Some word processor and spreadsheet documents can also have viruses hidden in them.

McAfee Anti-virus
Symantec (Norton) Anti-virus
eTrust EZ Anti-virus
Norman Anti-virus
Firewalls

A Personal Firewall is a program that watches and controls incoming and outgoing traffic to the internet, and makes sure no-one else is able to "hack into" your PC over the network. They also protect against "trojans" - programs that provide a "back door" into your computer - by only allowing programs you specify to access the internet.

ZoneAlarm - A great firewall program for Windows. Hides your open ports to prevent hacking, and offers powerful controls to prevent unauthorized software from accessing the network. Can be frustrating as you initially "teach" it which programs to trust, but will eventually run in the background with no fuss.
Blackice Defender - Well known commercial personal firewall, especially good at protecting against network hacking.
Resources
Gibson Research - Firewall news
How Viruses work
How Firewalls work
Virus Hoaxes - Virus hoaxes can be worse than the viruses themselves!
McAfee Virus Information Library
Advertisement Removers

If you get sick of seeing adverts and pop-ups on web pages, the following programs can help to get rid of them.

Naviscope - Great ad-remover and web accelerator for Windows.
Junk Buster - Ad-removing proxy for Windows and Unix.