If the seller proclaimed that he/she was selling a working computer, strongly suggest that you get your money back. You don't know what could be wrong nor the real condition of the computer.
Macs are not suppose to be sold w/o the system DVDs that originally came w/the computer. You will need to obtain them from the seller. System DVDs are machine specific and cannot be used from one computer to another. You also need them for troubleshooting as one of them comes w/the Hardware Test.
If you're not in the US, you may need to go through the regional Apple Store that serves your location to find the contact number. Here's a list of links to all of those - http://store.apple.com/Catalog/US/Images/intlstoreroutingpage.html Another resource: International Support Phone #s.
1) Try a safe boot.
Shutdown your machine. Hold down the shift key. Poweron. Wait awhile Wait awhile while you harddrive
is being checked.
2) You need to get into single use mode for steps one and two that are listed below.
This page will tell you how to get into single user mode.
Basically, you hold down the command + s key then powering on your machine. The command key
has a little apple symbol on the lower left. It is between the alt/option key and the space bar.
On a PC keyboard, it will be the windows key, I think.