Hi,
You are correct, only available option is through registry settings.However definitions will be stored on the root drive i.e ( C drive). It's by design & can not be changed.
The virus definition folder that is located in C:\Program Files\Common Files\Symantec Shared\VirusDefs (Windows XP path)
Following path also can not be changed:
Windows 2003: C:\Documents and Settings\All Users\Application Data\Symantec\LiveUpdate
Windows 2008: C:\ProgramData\Symantec\LiveUpdate