this is for tips and tricks, and tutorials, not questions, this should be in System and Software questions.
to answer your question, i would reccommend wiping and starting again. put the install disk in and reboot holding 'c', the reinstall Mac OS X using the 'Erase & Install' method.
there is no telling what the previous owner did, so to be sure, and safe, and for an overall better experience, always start wqith a clean machine.
oh, and remember to back up any documents/media that you've done since you bought it.