is this software efficient? and it can scan all the system files of OS?Quote from kiwi_trader:
Antivir is free like AVG for personal use but has had fewer reports of dissatisfaction with speed or false positives.
If you don't want to run real time monitoring the Open Source communities Clam is a good option.
If you do get a possibly false positive then try submitting the file to this site which runs most of the major checkers on it and lets you know which ones don't like it.
http://virusscan.jotti.org/en