System does not shutdown properly

My new dell 531 (I know…) with Vista home premium does not always shutdown (or reboot), often hanging up, which means I have to power off.

I have loaded all the updates (problem?) for Windoze as well as Office 2003, which is the only App that i have loaded….

well, I have Norton 360 loaded, but I disable the Vista firewall and using the Norton instead.

Any ideas?
Thanks,
Jerry