I don't know if this has been posted or not. But I'll just do it anyway. These programs are free and effective (in my experience anyway).
Spybot-Search and Destroy
Anti-Spyware
http://www.safer-networking.org/en/index.html
You guys will probably know about this one by now. It detects and remove most of the well-known malicious dialers, key loggers and suspicious web pages.
Ad-Aware SE Personal
Anti-Spyware
http://www.lavasoftusa.com/software/adaware/
Yet another well-known free anti spyware program. It is safe to say that Ad-Aware detects what Spybot missed out (and vice-versa). For extra precaution, install both of them.
Microsoft Anti-Spyware Beta (credits to Shinsengumi)
Anti-Spyware
http://www.microsoft.com/athome/security/spyware/software/default.mspx
Yep, this is an official Microsoft product and I heard it is pretty promising. Beware that this is the beta version. Don't keep your hopes up on this one.
CWS Shredder
Anti-Spyware (Cool Web Search remover)
http://www.intermute.com/spysubtract/cwshredder_download.html
I came across this Cool Web Search spyware program and it hijacked my Explorer and messed up with the registry. If you guys have any problems in dealing with it, use this to remove it.
AntiVir Personal Edition
Free anti-virus software
http://www.free-av.com/
The best...anti-virus program...ever...The real time scan took only a minimal RAM amount and the virus definitions updates almost every 3 to 4 days. Not only that, it can even detect Backdoor virus and some of the trojan files. Give it a try.
McAfee Avert Stinger
Free anti-virus software (worms)
http://vil.nai.com/vil/stinger/
This piece is not good for overall computer protection but its effective in removing the dastardly worms variants such as Netsky and Blaster. Combined with AntiVir, this is one serious tool to have.
Zone Alarm
Firewall
http://www.zonelabs.com/store/content/home.jsp
If you're using the built in Windows firewall, DON'T DEPEND ON IT. Zone Alarm is one of the best free firewall out there with various settings and customization.
Sygate Personal Firewall
Firewall
http://www.sygate.com/
If for some reason, you have nightmare using Zone Alarm (like me), you can try Sygate free firewall. Despite lack of customization compared to Zone Alarm, Sygate still hold its own against its rival. This is an alternative to Zone Alarm.