Hello,
By design, the Virus definitions are installed on the C drive.
As far as Sylink.xml is concerned, check this Article:
What is the location for Sylink.xml in Symantec Endpoint Protection 12.1?
http://www.symantec.com/docs/TECH165055
NOTE: Be sure to configure Windows Explorer to show hidden folders.
sylink.xml for SEP 12.1 (win 2003):
C:\Documents and Settings\All Users\Application data\Symantec\Symantec Endpoint Protection\12.1.1000.157.105\Data\Config
sylink.xml for SEP 12.1 (win 2008):
C:\ProgramData\Symantec\Symantec Endpoint Protection\12.1.1000.157.105\Data\Config
Hope that helps!!