Helpdesk rights to change passwords

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I have several differrent customers that have a single Windows 2003 DC
with XP SP2 workstations. At each site, I'd like to take one person
and give them rights to change/reset other user's passwords from a XP
SP2 workstation. I've found the following commmand:

net user username password /domain

which works fine when run from an account with domain admin rights.
When I run this as a standard domain user, obviously I get an error.

I've run the delegation of control wizard, picked the account I want
to have these rights, created a custom task to delegate, select an
active directory object type of users and selected both change
password and reset password.

When running the above command as the user I've delegated as the
password changer, on that person's workstation, I get:

System error 5 has occurred.

Access is denied.

What am I doing wrong, and what is the best way around it? I only
want this person to be able to change or reset passwords...

dcc
.