there is also the Quick scan option which would be the as its name implies be the quickest, but that scan doesn't do a rootkit scan. Whilst the standard and thorough scans do a rootkit scan as a part of the on-demand scan (the rootkit scan should take seconds rather than minutes, so shouldn't impact in the overall duration).
Ok great, i think it would be better doing the standard scan that does rootkit scan, thanks again for the info