Administrative Shares not working

Hello,

I'm using Windows Vista RTM in Addition with OneCare and activated UAC.
I created a user in the group "Administrators" with a password.

I'm trying to Access my Vista System from another machine in my network with
windows xp sp2 using the administrative shares c$ etc...
These shares are activated and visible in computer management.

When I try to access these shares from the xp machine a window pops up
asking for credentials. I enter the credentials of the account I created but
the window pops up again empty after pressing enter, so the credentials seem
to be invalid.

I created manual an additional share of the volume C with security and
sharing permission full control for the group of administrators and I can
access this share with the same credentials without problems.

How can I use the administrative shares?

thank you,
Shrek