Malicious script added in my website automatically

My website get infected by some malicious script My site is in Asp.net and i have used master page, some unknown script tag added outsite of content place holder.And my site gives error. Is this a virus attack or any malware. I have removed many time but when i uploaded pages after sometime my site get infected with those scripts Please suggest me to get rid of this problem. I have also scanned my machine with AVG antivirus but problem is still there Which is the best antivirus software to protect thesre kind of attacks Please help me.. Here is the error Server Error in '/' Application. -------------------------------------------------------------------------------- Parser Error Description: An error occurred during the parsing of a resource required to service this request. Please review the following specific parse error details and modify your source file appropriately. Parser Error Message: Only Content controls are allowed directly in ...