EDIT: JUST REALIZED HOW OLD THIS THREAD IS. PROBABLY NOT AN ISSUE ANYMORE, DON'T YOU THINK?
First of all you are doing a great job using MalwareBytes! It's a great program and I can't tell you how much help it has been in removing unwanted junk from family members' computers.
As far as a free antivirus, I recommend AVAST! Antivirus Free edition. In addition to scanning the PC for viruses, it has several real time shields that prevent malicious files from being downloaded in the first place. Included is a file system sheild, web shield, network shield, and P2P shield to name a few. Avast also offers the ability to virtualize processes, hence shielding the rest of the system from potential damage.
Definitely check it out! It even talks to you! :O