Good day Gentlemen,
Thank you for your replies.
Eddy, I have reinstalled avast and it is now working properly.
Nevertheless, I just like to share that prior to the reinstallation, when I first powered up my PC yesterday I could update virus definitions but could not run Quick and Full scans. I was able to run Boot-time scan, though. Also, the real time shields got disabled by itself.
Coincidentally, my PC was also acting very sluggish - very slow bootup and shut downs. Thus far, this seemed to be resolved after the reinstallation of avast.
Thank you All once again. Cheers.