the C$ and ipc$ shares

Hi pen-testers et al,
I am new to the whole 'pen-testing' science and am trying to work through the 
book Hacking Exposed 5th ed (alas, the going is slow because Computer Org and 
Database class takes up my time). I have been testing the "null session" 
vulnerability on Windows machines. I am able to connect to the IPC$ share of a 
computer and enumerate users (using enum.exe), I can use Sid2User and User2Sid 
to do the same, I have also had success using the command: net use * 
\\xxx.xxx.xxx.xxx\SharedDocs to connect to the Shared Folders of a machine. The 
question that I pose is two-fold: 1) as a pseudo/learning pen-tester, when you 
are connected to the SharedDocs folder is there anyway to delve further into a 
system? 2) If I can connect to the ShareDocs and IPC$ shares of a computer 
using the user name of "x" and a password of "" (null), why can't I do the same 
with the C$ share? Is this because the SharedDocs share is in the group 
'everyone'? Thanks alot guys and happy coding! 

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