hello there,
the advancedxpdefender infection might be on the computer that hosts your website. if i needed to place a bet, i'd say the host computer is infected, but there's also several other possibilities.
you say you've removed the malicious code from index page, but it might be also inserted anywhere else in the website code. you should look through the code really carefully; i know it might take a lot of time, but it's worth it.
there's also a possibility that the infection is on the computers that are used to develop the website. in this case, everytime someone opens website code from the infected computer, advancedxpdefender sneaks into websites code. either developers computer or host computer is infected, the infection might reach the machine through network connections, so ALL the computer in the network may need to be scanned for parasites.
have you changed the passwords for accessing websites code after the infection appeared? if the attackers have your passwords, they can access the code remotely and do whatever they want.
you can search for malicious files using
advancedxpdefender removal tutorial, you can also scan computer with
free antispyware software. if the host computer belongs to third party company, you should strongly recommend them to take this issue seriously.
hope this helps and i hope to hear from you soon.