Clamscan detested a virus in Microsoft Internet Information Services 8.5 log file:

C:\inetpub\logs\LogFiles\W3SVC1\u_exNNNNNN.log: Php.Trojan.MSShellcode-81 FOUND

I looked at the file manually, it consists of comments and GET and POST messages. How do I determine if  this is a real or false positive? The files are dynamic and new files will be generated, how are my options? Thanks.