The Anti-malware testing group AV-Comparatives.org tested sixteen products on the malware removal/cleaning capabilities of the security products installed on an already infected/compromised system.
The best performing free software by the Anti-malware testing group was Microsoft Security Essentials and they've also given it their best ranking in their performance test as well.
I have installed Microsoft Security Essentials and have found it to be very fast and light on system resources. I compared it on a PC running XP Pro SP3 machine with AVG 9 Free edition with Spybot Search and Destroy. I have now removed AVG 9 Free Edition and Spybot Search and Destroy and replaced it with Microsoft Security Essentials. The PC is running faster and has more free memory!
To download this free tool Microsoft Security Essentials Click Here
For additional information see:
AV-Comparatives picks six malware removal winners
Performance Tests [AV-Comparatives]
AV-Comparatives Rates Anti-Malware Performance [PC Mag via @edbott]