encyclo - 10:45 pm on Jul 27, 2009 (gmt 0)
There can be any number of server-side scripts which, if not updated, can cause problems. Do you have any content-management system or blog script installed? If you don't, another possibility is that your FTP credentials have been compromised - there is a worm variant going the rounds which infects your local machine and steals FTP passwords from programs such as FileZilla (used for FTP transfer). So, check your local machine too.