iFrame Injection

My website was recently attacked by <iframe> injections. As a result Google put it into the list of suspicious websites. On searching the internet and inquiring from my webhost, I came to know that some malicious malaware was using my ftp access (from the computers I used) and was replacing the index.htm/php files of my websites with an <iframe> containing page.


I use one PCs for my ftp access. ( have XP professional installed). The PC has full functional Zone Alarm installed. I thought that the malicious malware may be in my PC . So I sent this PC for a complete reformatting and fresh install to remove all instances of this malware. As a further protection, I run a full system scan of my home PC using Zone Alarm. After doing this, I was hoping that this problem was solved.
I changed FTP ,MYSQL,Admin user password of my site as it was recommended as the solution of this attack

However, just 1 days my website was again banned by Google for spreading a malicious virus. This time a different <iframe> Injection was used to insert the malicious code in all the index pages.
i keep watching my pages and daily index.aspx page are changed with diff iframe
I search a lot to solve thios problem but could not find any solution
Some other static sites are also hosted on same server i am using and all other sites are running correctly which shows that server is not effected
Please help me what should i do