Spybot Search&Destroy. This one is absolutely great. Not only does it remove many harmful types of software from your PC, it can also immunize browsers like IE and Firefox to known malware and malicious sites.
[URL="http://www.safer-networking.org/en/spybotsd/index.html"]http://www.safer-networking.org/en/spybotsd/index.html[/URL]
Super-Antispyware. The name sounds cheesy, but the results are great. This will also find and remove many viruses if you don't have good antivirus software on your box.
[URL="http://www.superantispyware.com/downloadfile.html?productid=SUPERANTISPYWAREFREE"]http://www.superantispyware.com/downloadfile.html?produ...[/URL]
Malwarebyte's Anti-Malware. A third tool in the arsenal, because even the previous two tools don't always catch everything. It can be found here:
[URL="http://download.cnet.com/Malwarebytes-Anti-Malware/3000-8022_4-10804572.html?part=dl-10804572&subj=dl&tag=button"]http://download.cnet.com/Malwarebytes-Anti-Malware/3000...[/URL]
Lastly, if you are fairly tech savvy, you can get an in-depth report on what's running on your machine to help manually identify any potential issues using HiJack This. It's good, but takes some patience and knowledge to utilize.
[URL="http://www.hijackthis.de/"]http://www.hijackthis.de/[/URL]