Can't connect to \\hostname\C$ as Administrator

Hi all,

I'm a technical admin on a network of around 300 PCs. Some are on a domain,
some are not, but all pretty much share the same Administrator name and
password for consistency.

Generally I can browse to Windows machines by entering \\hostname\C$ into
the 'RUN' prompt. Eventually, I will be asked to provide a name and
password, for which I enter the Administrator's credentials. This works
well, and allows me to automate backups from a linux server.

One PC (Windows XP Pro, not on the domain but with the same Admin
credentials) doesn't want to cooperate and I can't fathom why. When I
connect to it using the above method it responds with the username requester
box, but the actual username is fixed as 'Guest' (greyed out, rendering it
unchangeable). I have tried re-running the sharing wizard on the particular
PC, checking the admin account is working and turning off the firewall but
it still insists on only allowing this 'Guest' user.

Other XP machines (also not on the domain) I have tried allow me to enter
the Administrator user and grant access to the drive.

Can someone offer some advice on how to access this particular PC as Admin?
As this is a new venture, I expect I am going to encounter more PCs with
this behaviour.

Thanks in advance.

Elliot