Website Security Oscommerce

Im looking for someone who has experience in website security for Oscommerce. My site was hacked with a redirection that takes customers to a fake antivirus scan when they clicked on the categories. The site is now working but I havent been able to clear out the hacked files entirely. If the htaccess file in the root directory is modified in any way the site begins redirecting people again. I need someone to clear out the offending files - and any others that might be lurking and to safe guard the htaccess file from future attacks, preventing people from accessing, downloading or otherwise compromising the site. I would also like the site checked for other potential security flaws such as file permissions and such website is [login to view URL]