Virus Found in Message information

Hi All,

I got this Message from one of my mail users very frequent,but the problem is virus scans on his computer show no threads.I also did some scan in safe mode and the same result. What can be happening here.Any suggestions

This is the message below.
Symantec AntiVirus found a virus in an attachment you (nicholas_gill@goddent.com) sent to taiyo@acelerate.com.

To ensure the recipient(s) are able to use the files you sent, perform a virus scan on your computer, clean any infected files, then resend this attachment.

Attachment: Information
Threat: W32.Netsky.P@mm!enc
Action taken: Delete succeeded
File status: Delete