The following products are free for home users.

Antivirus
http://free.avg.com/
http://www.avast.com/
http://antivirus.comodo.com/

Microsoft Safety Scanner
http://www.microsoft.com/security/scanner/en-us/default.aspx

Online Virus Scanners
http://virustotal.com/
http://virusscan.jotti.org/

Firewall
http://personalfirewall.comodo.com/

Secure PDF Viewer
http://www.foxitsoftware.com/products/reader/

Spyware Removal
http://malwarebytes.org/
http://www.safer-networking.org/en/home/index.html
http://www.microsoft.com/athome/security/spyware/software/default.mspx

Security Tips
http://www.microsoft.com/protect/default.mspx
http://www.antiphishing.org/consumer_recs.html

Detecting and Removing Conficker Worm
http://support.microsoft.com/kb/962007