Over the past few weeks we have seen a few cases of trojans capturing ftp passwords from a customer's computer and use those details to modify their web site.
This may be through either a keylogger or access to an ftp program's saved account details (such as in FileZilla).

We recommend using an up-to-date virus scanner along with regular scans using a dedicated malware scanning program.

Avast! Home Edition is a free virus scanner, while Malwarebytes Anti-Malware has proven to be an effective spyware scanner.