yongsua, you can run the free versions of both MBAM and Emsisoft Antimalware, like I do. They are both demand scanners and have no real time protection. The Ikarus Antivirus engine in Emsisoft is great as a second opinion to Avast. Still, I turn off the real time scanners in Avast before running any other scanner just to avoid possible conflicts.
With Emsisoft you have an additional choice of using an installed program (Emsisoft Antimalware), or a portable version with no services or other changes made to your system (Emsisoft Emergency Kit). The latter, and the one I use, also includes a few bundled antimalware utilities like BlitzBlank and HiJack Free.