How to stop virus?

Hi,

We were using Opencart 1.4.3 before and our website got attacked where some code like this was being entered into the index.php file:
and also the .htaccess file was altered. We then upgraded to 1.4.9.6 and didn't experience any problems for a month or so, but now the virus has returned and the same problems are happening. When the above code is left in index.php then anyone who visits the website is automatically redirected to a website containing a trojan.

How can I fix this and is there a vulnerability is 1.4.9.6 that can't be fixed?

Thanks for any help

Do a clean install and/or Upgrade your OC site to the newest version.

Change your passwords (admin, cPanel, database, etc.)

Password protect your admin directory with .htaccess

Clean your computer with some kind of virus scanning program.