If you’ve gotten a computer with lots of useful software still installed - and it sounds like that’s what has occurred - then as a first step you’ll want to read my tutorial on How to Change the Owner Name on a Mac.

That’s why I encourage people to quite literally start from scratch by reinstalling Mac OS X if they’re buying a used computer, whether from a private party or a company.

Data about the user just seems to infiltrate all the nooks and crannies of the OS. Unfortunately, like many different computers, Macintosh systems are easier to set up from scratch the way you want than to tweak and alter after it’s been used for a while.