computer useless?

[macnewbie007]

Hi guys.I bought a second hand mac. I turn it onand it asks me for the login and password. I tried to contact the man but hes yet to return my emails. The computer seems legit as it has all the invoices etc.
What can i do to login? will it fix it if i instaled the new os x ?
ANY help will be very much appreciated.
thanks in advance guys.

 

[rman]

If you are running OS X and have the install media, you can either reset the pass word or install a fresh/clean operating system.