iframe malicious script - Mac - FTP compromised

Problem: A couple months ago I some how got some malware on my mac at work (I don't know how because I don't go to "the sites you should stay away from", so I am guessing it was probably from some stupid ad banner. ANYWAY, when I logged into our website via FTP, a week later I noticed our website was blocked by Google (a client actually called us to tell us) and so I looked around to find the malicious script hidden iframe. The problem is it doesn't affect my mac. I'm getting tired of removing the script from our website every other week not to mention I have no idea what the affects are on a mac and don't want to compromise other peoples computers.

FTP Software: FireFTP
Mac OS: 10.4
Software tried: MacScan (did not detect any malware)

I am trying to take care of this without involving any money and unfortanately there isn't much "free" Software out there for Macs or scanners.

Anyone know how to remove this? I am having the FTP passwords changed but I do not want to log back into the FTP only to have it happen all over again and then have to change our FTP passwords again.

Please help!
Thank you.