Donna Buenaventura, a highly respected security software expert and MVP, wrote a comparative review on BrightHub.com:

Which is Better? Malwarebytes’ Anti-Malware vs. a-squared Free

Her test results:

a-squared Free won in the major categories!

• Malware Detection – Winner: a-squared Free
  Of 100 malware samples, a-squared Free found 92, Malwarebytes’ Anti-Malware only 58.
   
• Removal of Detected Malware Samples – Winner: a-squared Free
  a-squared Free was able to remove all detected samples without the need for a system restart.
   
• Performance – Scanning – Winner: a-squared Free
  a-squared Free finished a full scan of the system drive in 29 minutes, Malwarebytes’ Anti-Malware took 1 hour and 24 minutes.
   
• Options and Feature – Winner: a-squared Free
  a-squared Free lets you run a quick, smart, custom, right-click, silent, commandline and deep system scans.

The famous Malware Research Group has again tested 21 antivirus products to protect your PC in October 2009. Tested virus scanners had to find the enormous amount of 554,891 malware samples, split into the following categories:

Trojans/Backdoors: 398,951
Windows Viruses: 8,864
Worms: 61,928
Adware/Spyware: 48,552
Rootkits/Exploits: 10,736
Other Malware: 25,860 

That ensures a representative mix of current malware types that is similar to a ”in-the-wild” malware mix. Test enviroment was a Windows XP machine with service pack 3. Here are the original test results:

We are proud to be able to defend our number one place!