In addition to a firewall you must have full time (active) virus protection and full time (active) spyware protection.
All the following programs are free.
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Full Time (Active) Virus Protection
If you do not have active virus protection, install only one. All are excellent.
AVG Antivirus 7.5 Free Edition
http://free.grisoft.com/freeweb.php/doc/avg-anti-virus-free/lng/us/tpl/v5
http://www.download.com/AVG-Anti-Virus-Free-Edition/3000-2239_4-10669237.html?tag=lst-0-1
or
Free antivirus - avast! 4 Home Edition
http://www.avast.com/eng/avast_4_home.html
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Full Time (Active) Spyware Protection
Start with Windows Defender.
Windows Defender
http://www.microsoft.com/athome/security/spyware/software/default.mspx
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Install the following four programs and run weekly or at least monthly. You need all four. They will greatly increase your protection. They are not a substitute for full time spyware and virus protection.
Ad-Aware SE Personal (update + full scan)
http://www.lavasoftusa.com/products/ad-aware_se_personal.php
Spybot Search & Destroy (update + immunize + scan)
Do not enable Tea Timer and SDHelper
After installation: update + scan + immunize
http://www.safer-networking.org/en/mirrors/index.html
SpywareBlaster: Update then open and click “enable all protection”.
http://www.javacoolsoftware.com/spywareblaster.html
SUPERAntiSpyware free version: (update + scan)
http://www.superantispyware.com/
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Install The following:
CCleaner: Do not install toolbar option
Removes tracking cookies, unneeded files, history
In options.
Set to run when computer starts.
Place cookies you want to keep in save list
http://www.ccleaner.com/
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Install either one not both:
McAfee Site Advisor: Internet Explorer and Firefox
http://www.siteadvisor.com/
McAfee SiteAdvisor helps protect you from all kinds of Web-based security threats including spyware, adware, spam, viruses, browser exploits, and online scams.
TrendProtect™
http://www.trendsecure.com/portal/en-US/free_security_tools/trendprotect.php
TrendProtect is a FREE browser plug-in that helps you avoid Web pages with unwanted content and hidden threats.
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Monthly Run
Microsoft Update:
Run in "Custom Mode". Install everything, reboot and repeat until nothing is left to install.
http://www.update.microsoft.com/microsoftupdate/v6/default.aspx?ln=en-us
Microsoft OneCare Live, run “full service scan”
Updates windows, virus and spyware scan, disk cleanup, disk fragmentation (if needed), backs up registry and then cleans registry, and checks for open firewall ports
http://onecare.live.com/site/en-us/default.htm
Malicious Software Removal Tool (run “full scan”)
http://www.microsoft.com/security/malwareremove/default.mspx
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Online Free Scanners:
Run Trend Micro, Kaspersky, and Panda Scan now.
Run a different one each month.
Avast! Online Scanner
http://onlinescan.avast.com/
BitDefender Online Scanner
http://www.bitdefender.com/scan8/ie.html
CA eTrust® PestScan
http://pestpatrol.com/pestscan/index.htm
eTrust Antivirus Web Scanner
http://www3.ca.com/securityadvisor/virusinfo/scan.aspx
F-Secure Online Virus Scanner
http://support.f-secure.com/enu/home/ols.shtml
Kaspersky - Free Online Virus Scan
http://www.kaspersky.com/virusscanner
McAfee - Free Scan
http://us.mcafee.com/root/mfs
Panda Software - ActiveScan
http://www.pandasoftware.com/activescan/activescan/ascan_2.asp
Symantec (norton) - Security Check
http://security.symantec.com/sscv6/default.asp
Trend Micro™ HouseCall
http://www.trendmicro.com/hc_intro/default.asp
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Rootkit Removers
Pick any 2 and install and run one each month.
AVG Anti-Rootkit
http://www.grisoft.com/doc/products-avg-anti-rootkit-update-app-art/?ver=1.1.0.29
F-Secure BlackLight
http://www.f-secure.com/blacklight/
Panda Anti-Rootkit
http://www.pandasoftware.com/products/antirootkit/
Sophos Anti-Rootkit
http://www.sophos.com/products/free-tools/sophos-anti-rootkit.html