UAC is set up to not allow access to default shares remotely. To enable, create the LocalAccountTokenFilterPolicy DWORD value at this key in the registry
0 – build filtered token (Remote UAC enabled)
1 – build elevated token (Remote UAC disabled)
By setting the DWORD entry to 1, you will be able to access the administrative shares since the remote logon token will not be filtered.
This is discussed in this KB article, http://support.microsoft.com/kb/947232. (It’s for Vista but it applies to Windows Server 2008 R2)