Struggling to install SBS2003 R1 SP1

Hi there,

I've been experiencing a problem trying to upgrade a client's SBS 2003 R1
server to SP1.

First off, we have a GP in place to change the administrator account on all
computers in the domain to <AdminName>. Thus I log into the server as
<Domain>\<AdminName> via TerminalServices (it's a headless server). The
username at the top of the Start menu shows administrator, so all appears fine
- I'm logged on as the built-in Administrator.

Shortly after the SP1 upgrade process begins an error message is displayed
stating that you need to be a member of the Domain Admins, Enterprise Admins
and Schema Admins groups to install the update. This is followed by another
message offering to display the setup.log file from C:\Program Files\Microsoft
Integration\Windows Small Business Server 2003\Logs.

There are no errors listed in this log file, all modules are listed at
installed successfully.

I have confirmed that the Administrator account for the domain is a member of
Domain Admins, Enterprise Admins and Schema Admins groups.

I've unsuccessful so far in finding an explanation for this. Any suggestions as
to how I can solve this issue?

Thanks,
Justin