It might be better to try mounting the share via the "Map Network Drive" option than to run it directly from the "Run" prompt. Choosing the former allows you to specify the domain, username, and password; whereas the latter uses the account credentials with which you're currently logged onto the local machine.
If that doesn't help, you might try editing the properties of the C$ share through Computer Management. To do this (although you probably already know this), right-click on My Computer and choose "Manage", and navigate to Computer Management >> System Tools >> Shared Folders >> Shares.