Thou must use anti-virus/firewall software and update it regularly

Given the rise in computer threats (viruses, trojans, malware, greyware, spyware), it is an absolute must to have proper protection (aside from practicing smart computer habits).

Kapersky labs and Symentec have released it's findings for the year 2008/2009 (see notes at bottom), and have reported that the vast majority of threats on the web are predominantly trojans (programs that present useful function, but also contain undisclosed and malicious components - http://en.wikipedia.org/wiki/Trojan_horse_(computing).

Fortunately, these threats can for the most part be countered by maintaining proper anti-virus protection (conducting daily updates and weekly scans). There are a multitude of anti-virus products, some of which are tuned towards centrally managed (mostly for the corporate side) and for individual users.

There are antivirus offerings that can be very expensive, suites that are available freely to the consumer, and free web based scanners.

Cautionary note: most new machines come pre-loaded with anti-virus programs, but they usually only offer free support for a limited time.

Here's a list of free antivirus programs:

AVG - http://free.avg.com/
Clamwin - http://www.clamwin.com/
Avast - http://www.avast.com/eng/avast_4_home.html

Here is a link to reviews of the most popular antivirus applications from CNET (a reliable and very trustworthy source):

http://reviews.cnet.com/4566-3681_7-0.html?tag=leftColumnArea1.0

Here is a list of free, online virus scanners:

Trendmicro Housecall -http://housecall.trendmicro.com/
Kapersky online - www.kaspersky.com/virusscanner
Symantec - http://security.symantec.com/sscv6/WelcomePage.asp
Mcafee - http://home.mcafee.com/Downloads/FreeScan.aspx

I also use The Stinger as a stand-alone (free) virus removal tool - http://vil.nai.com/vil/stinger/

Good tip: If an infection is found, it is best practice to use multiple software suites for removal, as different antivirus programs can detect/remove infections better than one alone. Conducting research about the name of the virus can also direct you to removal instructions specific to the infection.

Last but not least, here is a few spyware/general cleaner utilities that I have found to be very useful:

CCleaner - http://www.ccleaner.com/ (cleares up registry entries, caches, temp files)
Lavasoft Adaware (free version) - http://www.lavasoft.com/single/trialpay.php - anti-spyware
Spybot Search and Destroy - http://www.safer-networking.org/en/index.html - like adaware

Notes:
http://www.kaspersky.co.uk/news?id=207575671 - Kapersky 2008 threat statistics
http://www.symantec.com/about/news/release/article.jsp?prid=20090413_01 - Symantec