I would start with downloading the latest Rapid Release definitions.
Once you have the new definitions, boot into safe mode and run a Disk Cleanup (right-click the C drive, Properties, Disk Cleanup) - that will delete all the files that are in these temporary locations, as well as IE's temporary files, etc. Perform a full system scan in safe mode.
If the above steps fail to detect and remove the threats, here are useful tools that Symantec provides to help with finding those hard to detect threats.
1. The Power Eraser Tool eliminates deeply embedded and difficult to remove threats that traditional virus scanning doesn't always detect.
2. The SERT (Symantec Endpoint Recovery Tool)is useful in situations where computers are too heavily infected for the Symantec Endpoint Protection client installed upon them to clean effectively.
3. The Load point Analysis Tool generates a detailed report of the programs loaded on your system. It is helpful in listing common loadpoints where threats can live.
Rapid Release Virus Definitions –http://www.symantec.com/business/security_response/definitions/download/detail.jsp?gid=rr
Support Tool with Power Eraser Tool included –http://www.symantec.com/business/support/index?page=content&id=TECH105414&locale=en_US
How to use the Load Point Analysis within the Symantec Support Tool to help locate suspicious files http://www.symantec.com/business/support/index?page=content&id=TECH141402