That's the trend nowadays.The virus if its new it will first attack the the antivirus and then download other virus that would have been caugh by symantec.
I would suggest you to Map the drive of this computer to any other computer with updated virus definitions and scan that drive.
If still nothing is detected.It might be a rootkit that is hiding from the User Level and sitting down in Kernel level.
Run Rootkit Revealer from Sysinternal or IceSword to find out if there is any rootkit.
You can also try downloading and running Norton Security SCAN tool and run a scan from that or run an online scan.