Essexboy, thanks for your prompt reply.

I have two questions before running the programs:
-As Pondus noted, I currently have installed and running: Avast, Norton Internet Security and Microsoft Security Essentials. Would it be ok if I uninstall the last two before proceeding with your instructions?
-I suspected of a usb drive as an infection alert popped up as soon as I plugged it a few weeks ago (because of the alert, I didn’t opened it or any of the files in it). I only have two files in the drive that I don’t really need. Would it be ok if I just discard it? Should I run the MCShield on an external hard drive that I regularly connect via USB to my laptop?