Hello,
First you can update the latest defintion and then update the latest microsoft bullet in.
After that scan the pen drive. Virus will be clean. If any virus related file will be still there then update that virus file to symantec security team. Below link for submission
https://submit.symantec.com/websubmit/platinum.cgi
https://www-secure.symantec.com/connect/articles/using-symantec-help-symhelp-tool-how-do-we-collect-suspicious-files-and-submit-same-symante