Urgent - Suspected Hacking - Please help!

We have an eCommerce store (which despite this issue is functioning at present as normal with no issues). We have just received a notification from surbl that our website has been blacklisted due to being likely cracked and hosting a spam, phishing or malware site. The following notification was included: "Your website has been blacklisted due to being likely cracked and hosting a spam, phishing or malware site. This often means that the web server is insecure, has been broken into, and malicious sites were placed on the web site. Be sure to also check existing web pages, which sometimes have malicious content added to them. It may also mean that the DNS control panel or DNS system controlling the domain has been cracked and is responding for a subdomain that is not on your server. In other words the bad guys may have set up their own subdomain under your domain by breaking into your DNS system. Please fully secure all computers, then explain in technical detail how they were secured. Please note that a common exploit is ftp password sniffing malware on the webmaster's computer. Therefore the webmaster's own computer may be infected. If so, it must be secured also. If it is not secured then any new passwords on any new servers, sites or hosts may also be stolen. Whenever possible, please do not use ftp. ftp is inherently insecure since it transmits passwords unencrypted. That makes stealing the passwords much easier. ssh, sftp, scp are better since they encrypt passwords, but can still be insecure if the computers they're used on are insecure. Please note also that simply deleting the malicious content and changing passwords probably does not secure the computer. The original security flaw that allowed the breach must be found and corrected. If it is not found, then the systems may still be vulnerable to exploits." We require you to please look at our site and rectify this issue in order that we can have our site taken off the blacklist and secured asap.