I would reccomend getting , AdawareSE
http://www.bleepingcomputer.com/forums/Using_Ad_Aware_SE_to_remove_Spyware_&_Hijackers_from_Your_Computer-tut48.html
Spybot (s&d)
http://www.safer-networking.org/
A good firewall is vital and this explains about them and offers a list
http://www.bleepingcomputer.com/forums/Understanding_and_Using_Firewalls-tut60.html
Spywareblaster will prevent many attacks.
http://www.javacoolsoftware.com/spywareblaster.html
Spywareguard too.
http://www.javacoolsoftware.com/spywareguard.html
Be careful with the cleaners, if in doubt, don’t delete, check first!!
Crapcleaner....
http://www.ccleaner.com/
EasyCleaner.....
http://personal.inet.fi/business/toniarts/ecleane.htm
Then you have to choose an Anti-virus
AVG....
http://free.grisoft.com/freeweb.php
Or AVAST....
http://www.avast.com/eng/avast_4_home.html
Or a-squared.....
http://www.emsisoft.com/en/software/free/
Or Computer Associates.....
http://www.my-etrust.com/microsoft/
For Trojans try these ( free to try),
http://www.ewido.net/en/
Or
http://www.misec.net/trojanhunter/
Or
http://www.agnitum.com/products/tauscan/index.php
Anti-spy
http://www.trendmicro.com/spyware-scan/
And always scan once a week with these free and trusted scanners as well if you go to gaming sites and porn and etc.
http://housecall.trendmicro.com/
http://www.webroot.com/land/freescan.php
http://www.kaspersky.com/virusscanner
http://www.pandasoftware.com/products/activescan/com/activescan_principal.htm
http://www3.ca.com/securityadvisor/virusinfo/scan.aspx
http://www.bitdefender.com/