Service pgsql-8.1 failed to start

I'm trying install PostgreSQL-8.1.10 (binary format) in an external HD (USB
connection) under Windows XP SP2.

1) If I try to install postgres as a service I receive the message
"Service failed to start; verify if you have sufficient privileges to start
system services"

I'm running the installation under an Administrative account.
I disabled AV program and Windows firewall.
In Control Panel/Administrative Tools/Local Security, postgres account has
permission to do logon as service.

2) When I tried didn't to install postgres as a service, the installation
finished and I could to run initdb and restore a small database. But, when I
tried start manually the pg_ctl
a) using the windows account
pg_ctl register -N "pgsql-8.1" -U postgres -P postgres -D E:\Data start
pg_ctl -D E:\Data start

the message was "You can start in an administrative account". Yes, I'm in
an administrative account, but I registered the service telling the user to
use...

b) without using the postgres windows account
pg_ctl -D E:\Data start
the message is "You can not start in an administrative account".
Yes, I'm in an administrative account...

I would like to keep an demonstration database in this external hard drive
and would like to attach this HD in different Windows PCs. Start manually
without change the user machine it is a better solution for me and
generally, the user is in an administrative account.

Any ideas?
Thank you.
Josi Perez