Which OS version? [the Leopard DVD has a special feature to reset *user* permissions.]
Anyway...
Someone gave my mom a computer but
... anytime anyone acquires a *used* computer, it's probably wise to boot from the system DVD and do a fresh install of the OS. [Backup the stuff on the HD to an external disk first, if restoring extraneous items left by the former owner might be of value.]
If this used Mac didn't come with any system install DVD or CDs (which seems to happen way too often -- whether the computer is hot or not), then buying the OS from Apple is another good idea.