Windows XP tells me "I may not have an antivirus program running", with this free antivirus installed. But, I can update the software normally - it tells me when it's out of date, and in the last few weeks I remember it alerting me to when it picked up a few viruses. What's the deal?
The free version of Malwarebytes does not have a real-time component. It is not actively monitoring your system. http://www.malwarebytes.org/products/malwarebytes_free When you first install, it runs as a trial (14 days) so real-time is enabled, but that has most likely expired. BTW: I use both the full version of Malwarebytes and Microsoft Security Essentials to keep my machine protected. However, I don't think MSE runs on XP. http://windows.microsoft.com/en-US/windows/products/security-essentials
I have been using Avast on Windows XP for many years. This website ranks Avast best and Microsoft Security Essentials second place: http://www.techsupportalert.com/best-free-anti-virus-software.htm