Problems with navigation....

Recently I 've instaled opencart 0.7.8,.... but when I'm in the frontend and I want to view a product I get this error from browser...

Not Found

The requested URL /opencart/product/product_id/1 was not found on this server.



What does it mean?

I just can navigate on index page... the rest of links doesn't work too...  :( , in the admin side, there are no problems with navigation...

what's wrong?
Please help me!!!

After reading some topics I found solution here!!!

http://forum.opencart.com/index.php/top ... ml#msg6829