There you have it, the gpupdate utility comes with XP. I assumed you have XP clients.

On 2000, you use secedit. Here is the syntax:

Refresh security settings
secedit /refreshpolicy
This command refreshes system security by reapplying the security settings to the Group Policy object.

Syntax
secedit /refreshpolicy {machine_policy | user_policy}[/enforce]

Parameters
machine_policy
Refreshes security settings for the local computer.
user_policy
Refreshes security settings for the local user account currently logged on to the computer.
/enforce
Refreshes security settings, even if there have been no changes to the Group Policy object settings.