I had a user with administrative permissions added onto a server 2008 system (on a workgroup) that could not map out the c$ admin share. I’m not sure of the exact error now since I’m writing this up about a month after the fact, but I found the fix.

Win7 client on domain connecting to win2k8 server on workgroup. Did the \username with password, still no dice. Registry fix fixed issue.

Download File
EnableCShare.reg

Install File

Wait about 30 seconds

Attempt C$ again

Obviously check ALL registry files BEFORE importing onto your system. Just in case someone tries to pull your leg.