I can test your server to check how was compromised, if was a Plesk fault or some of your websites. I`m a penetration tester specialized in web application security, also administrator of over 200 servers. I have to be sincerly, all of them *nix systems, but I`m familiar with Windows Servers too. I`m having at least one one compromised website per week infested with malware so it`s not new things for me. I can check your server security, based on that results we can take actions in order to patch the holes, remove malicious codes so your server can come back afloat. You can ask me anything before to prove that I`m able to do that.