The data collected during 2005 by the resonant bar Explorer are divided into segments and incoherently summed in order to perform an all-sky search for periodic gravitational wave signals. The parameter space of the search spanned about 40 Hz in frequency, over 23 927 positions in the sky. Neither source orbital corrections nor spindown parameters have been included, with the result that the search was sensitive to isolated neutron stars with a frequency drift less than 6 x 10(-11) Hz s(-1). No gravitational wave candidates have been found by means of the present analysis, which led to a best upper limit of 3.1 x 10(-23) for the dimensionless strain amplitude.