GPO update issues

For some reason when I change group policy I have to reboot the xp machines for it to take affect. I thought gpupdate would change it immediatly or am I missing something. Also It should update automatically every 90 minutes. I let it sit there all day and nothing updates. Is there something I'm doing wrong?
Comments
start
run type cmd
then type gpupdate in the command line......
I think things like startup scripts GPO settings would only run after a Reboot?
Do you have to reboot or is logoff/logon enough to get it to work? Maybe you could post up some of the GPO settings on here?