A cover or boot for use in conjunction with the nail advancement mechanism of a fastener-driving tool is disclosed. The cover or boot covers external surface portions of the nail advancement mechanism of the fastener-driving tool so as to prevent a significant accumulation of dust particles from effectively collecting upon and entering the piston-cylinder assembly of the nail advancement mechanism and thereby operatively interfering with the reciprocal movements of the piston of the piston-cylinder assembly of the nail advancement mechanism whereby the operative functioning of the nail advancement mechanism of the fastener-driving tool would otherwise be adversely affected.