How old is the computer?
Can you identify the OP system from the startup screen as Dias suggested?
Did you get a copy of the Operating System (probably Windows?) with the computer?
Is the seller still at the supermarket to ask him for the OP system software?
You should have received a copy of the OP System, but I suspect you may not have gotten the full package. It is pretty bad on any sellers part to sell a computer with the setup you describe. It will be difficult (or impossible) for the average person to fix the setup back to a familiar Windows environment.
Broni's suggestion:
Reformat?
is fine as long as you have a copy of Windows compatible with the computer you bought. By compatible, I mean this: If the computer is 10 years old, you need a version of Windows from the era. "Windows 98 SE" for example. Also, it would be good to have a friend who has reformatted and reloaded windows before to help you if this is your first time.
dahlarbear has a good suggestion that "might" work. Try it before you go with the full format and reinstall method.
Boot to the Windows Advanced Options Menu. From there you may select "Safe Mode". Use "msconfig" and selective startup to stop the "startup" programs from running. Reboot to normal mode and start figuring out which one of the "startup(s)" is the Bad Boy!
dahlarbear forgot to tell you how to get to the "Advanced Options Menu".
To get to that menu, turn the power on. wait about 2 or 3 seconds and press the F8 key. Repeat the F8 key once every second for the next 5-10 seconds or so and the menu dahlarbear described should come up. At that point select "Safe Mode". If you are successful in getting to that menu, there is a chance(slim) that you can salvage the OP system already loaded.
Try the suggestions given and give us the results.
AND let us know the answers to the questions I asked at the beginning of my reply.
The more information you give us the better chance that one of us will be able to help you.