I would not install AVG free because it lacks rootkit protection making it unacceptable. You may need to look if Trend Micro offers an uninstaller.
Prior to installing run the following:
COMODO Registry Cleaner (free) probably the best
http://download.cnet.com/Comodo-Registry-Cleaner/3000-2094_4-10895685.html
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Three Excellent Free Antivirus Programs
Only install one as installing more than causes problems.
Avira
http://www.free-av.com/
Avast
http://www.avast.com/eng/download-avast-home.html
Microsoft Security Essentials
New, looks very good and uses very little RAM.
http://www.microsoft.com/SECURITY_ESSENTIALS/
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Install and weekly update and run a full scan:
Malwarebytes' Anti-Malware free version
http://www.malwarebytes.org/mbam.php
SUPERAntiSpyware Free Edition
http://www.superantispyware.com/superantispywarefreevspro.html
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Alerts you if you visit a dangerous web site. Install only one.
McAfee Site Advisor (free)
http://www.siteadvisor.com/
Web of Trust (WOT) (free)
http://www.mywot.com/
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Make sure your Widows software is up to date. This is extremely important! Make sure to upgrade to IE8! Run "Windows Update" weekly.
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If you want to be 100% safe while surfing the web install Sandboxie. If will automatically set up to use your default browser. Per Sandboxie "Sandboxie runs your programs in an isolated space which prevents them from making permanent changes to other programs and data in your computer. "
In Sandbox settings:
Sandbox Settings > Delete > Invocations > Check "automatically delete contents of sandbox"
(see video below)
Sandboxie (free)
http://www.sandboxie.com/
Video of Sandboxie. Installation and use, (3 parts)
http://www.youtube.com/watch?v=GueXMq-Vyi8&feature=channel_page