Creating local users for a group of computers using GPO

That's because this is a group policy preference which has been introduced long after XP has been released. Having a W2k8 DC is not enough, you need to install the Group Policy Preference Client Side Extensions to be able to use them on XP.
Information about new Group Policy preferences in Windows Server 2008
http://support.microsoft.com/kb/943729

You should also upgrade those systems to SP3 anyway so you can get the security updates since July 13, 2010. You can't get any new XP related updates until you go to SP3.

http://www.daniweb.com/hardware-and-software/microsoft-windows/windows-nt-2000-xp/news/287954

You should be able to do this as is with restricted Groups policy

see here for more information

http://www.windowsecurity.com/articles/using-restricted-groups.html

Another way to do that without having to install the GPPCSE would be a computer startup script, for example as batch:

@echo off
net user "Serviceaccount" "NotSoSecretPassword" /add

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But that's a security risk, and it's something that you have to know is happening in this script, unlike the Preferences where you can directly see what's going on.

chakko,
Restricted Groups policies can't create users, neither local nor in the domain; they only control membership in/of existing groups and existing users.

I updated to XP SP3 and also tried in windows 7, but in both are not working..

I did the same step what is mentioned in the above article

Not sure why obda's suggestion didnt work. Should be a very simple way to do it....

you might try this to see if there is any error message
net user "Serviceaccount" "NotSoSecretPassword" /add > c:\output.txt

then check c:\output.txt for any message

www.kixtart.org for download and information