All of these are free and i use them and have done for some time, many of which I also recommend to resolve specific problems. They all have low resource usage, and my machine is kept free of viruses and malware apart from those that iI deliberately import to ensure that the programs do as the vendors claim:
Online Armor Free Firewall:
http://www.download.com/Online-Armor/3000-10435_4-10426782.html?tag=lst-1&cdlPid=10831167
Installation Tutorial:
http://cotojo.wordpress.com/2008/06/16
Avast anti-virus better definitions, anti-spyware and anti-rootkit built in, faster and more reliable:
http://www.download.com/Avast-Home-Edition/3000-2239_4-10019223.html?part=dl-AvastHome&subj=dl&tag=button&cdlPid=10852750
Setup Guide:
http://cotojo.wordpress.com/2008/07/15
Lavasoft AdAware2008:
http://www.download.com/Ad-Aware-2008/3000-8022_4-10045910.html?part=dl-ad-aware&subj=dl&tag=top5&cdlPid=10844457
How to use:
http://cotojo.wordpress.com/2008/06/01
Spybot S&D:
http://www.safer-networking.org/en/download/index.html
Download, install, update and Immunize, then click 'Check for problems' then when complete select all and then 'Fix Checked'
SpywareBlaster 4.1:
http://www.download.com/SpywareBlaster/3000-8022_4-10196637.html
SpywareBlaster prevents the installation of ActiveX-based spyware, adware, dialers, browser hijackers, and other potentially unwanted programs
SpywareGuard 2.2:
http://www.download.com/SpywareGuard/3000-8022_4-10514469.html?tag=pub&cdlPid=10514468
real-time protection solution against spyware
Also use Spyware Doctor Free Basic Edition, which offers realtime protection:
http://pack.google.com/intl/en/pack_installer_new.html?hl=en&gl=us&ciNum=12
Its free with Google pack, untick all boxes except Spyware Doctor. Download install, update and run.
KeyScrambler which encrypts everything that you type in including credit card numbers etc.
http://www.download.com/KeyScrambler-Personal/3000-2144_4-10571274.html
Works on most browsers, email clients and IM programs, and it's free.
How to use:
http://cotojo.wordpress.com/2008/06/06
Also install SnoopFree Privacy Shield which is not another scanner product. It's a powerful shield that guards your keyboard, screen and open windows from all spy software. The way it works makes it virtually impossible for any spy program to work on your computer:
http://www.snoopfree.com/default.htm
Defraggler:
http://www.filehippo.com/download_defraggler/
With many defragmenting programs you defrag the whole drive, but with this one you have the power to defrag specific files or folders as well as the complete hard drive if you want to.
http://cotojo.wordpress.com/2008/07/16/