When connecting to a Vista machine from another machine that is not on the same domain, Vista will filter the token.  You must disable Remote UAC in the Vista machine’s registry:

HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\
Policies\system\LocalAccountTokenFilterPolicy

   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.