The C drive is most likely to be shared by default as and administrative share. Assuming you are using XP.
You can mount the drive using the following in a command prompt:
net use z: \\computername\c$ /user:username
"z" being the drive letter you want to use.
"computername" is the name to the computer that is sharing the drive or IP address.
"username" is the user that has permission the access the drive.
You should then be asked for a password, just enter the password for the username entered.