These days, hackers are trying to attack websites by using I-frame Viruses. I have seen many websites infected by this deadly virus attack. If any site appears infected to Google, it sets a message “This site may harm your Computer” in the search engine results pages (SERP). Also, if you visit these sites, you will find a repetitive message “Reported Attack Site”!
Where from malware or iframe viruses come on a website?
So, how to remove such viruses from a website?
Generally, the virus attacks on index pages like index.html, index.htm, index.php or defalt.asp or home.html. You can easily find these codes on index pages, here is an example.

So, remove these codes from your infected pages. However, this is not a permanent solution. your web pages are likely to get infected again.
To remove it permanently you need to change its FTP password immediately.
Why malware or iframe viruses come back on a website again?
Virus attack cannot be removed completely. you are likely to get virus attack as soon as hackers will find your password. I too have experienced the same when my site was attacked. According to me, they hack the passwords through cracking security applications like IE, Adobe readers, Mozilla Firefox or vulnerable ftp.
To restrain your we pages from unwanted virus attacks, you need to keep a close eye on your system’s applications. Look for updates like recent Firefox Browser security and stability update etc. to increase the capacity of your system.