Hello,
Here are few Suggestions which would help you get the Disk space back as well as get rid of the Threat (if there is any..)
1) Remove the machine from the Network.
2) Now, since there is No Disk space as of now.. so it becomes very important to understand what is consuming the highest Disk space.. To figure out where all the space on a given volume went, WinDirStat is the tool I use to fight that problem.
The program scans one or more volumes and generates a diagram called a treemap that indicates which directories are consuming the most space. One glance and you’ll be able to discern which directories are the most crowded, which large individual files are needlessly devouring space, and what types of files constitute the biggest space-eaters. From there, you can open instances of Windows Explorer into directories you want to prune. You can also remove files and folders from within the program if you so desire.
3) Follow the steps in the Article, which would help you grab more space in case the Symantec Endpoint Protection is taking to much of Disk space :
https://www-secure.symantec.com/connect/articles/issue-related-low-disk-space
4) Once, you have the Disk space, you can Run the Symantec Endpoint Protection Recovery Utility (SERT)
http://www.symantec.com/docs/TECH131732
The Symantec Endpoint Recovery Tool (SERT) is a bootable CD that can scan and remove malware from an infected computer. SERT is useful in situations where computers are too heavily infected for the Symantec Endpoint Protection client installed upon them to clean effectively.
5) If Symantec is not detecting the Threat, it is adviseable to work on the steps provided in the Article below:
https://www-secure.symantec.com/connect/articles/using-symantec-support-tool-how-do-we-collect-suspicious-files-and-submit-same-symantec-sec
Hope this Steps would help you!!!