BleepingComputer.com: Accessing Windows Administrative shares - need help

I recently got a new laptop with Windows Vista Home premium x64, and I have a desktop with windows xp media center SP3
So I managed to get the computers on the same workgroup so that I could use file and printer sharing, and it works fine. But, I quite often find myself wanting to access other folders on one computer from the other, and I am getting tired of having to go on the other computer and right click on the folder blah blah blah.

A couple of days ago, I found out that by default, windows shares the root of all drives as an "administrative share" with a name like c$
so I go onto my computer and I type \\DESKTOP\c$ (the desktop is named DESKTOP and the laptop is named LAPTOP) but it comes up with a username and password box. Neither of my computers have passwords on the user accounts (and there is only one account on each computer) so i type HP_Administrator (the username that is on the desktop) and leave the password field blank and I get a message that says something like "DESKTOP\c$ is not accessible" and then something like "check with your network administrator to see if you have permissions"
So I try typing in Administrator into the box and I still have the same problem. is there a default Administrator password that I have to use?

So I want to know, how can I use the administrative shares that windows automatically sets up to get to the root hard drive of the other computer.
Even if I can only use the XP one (because vista seems to have a tendency to lock everything out and disable lots of advanced features in the home versions)

Thanks for any help you can give me