Summary:
My machine hangs on shutdown/restart: all windows and the menu bar disappear but the desktop wallpaper remains, and it stays like that without disk activity forever (hours). It doesn't even show the shutdown screen (the one with the animated dots) where I could hit ESC and watch the shutdown text. How can I troubleshoot this?

Details:
I've just received a new nettop computer (Acer Aspire Revo 3700: CPU:Atom D525, GPU:Nvidia ION2). I've just made a clean install of Ubuntu 10.10 using the standard USB pendrive method. The machine boots okay and works OK including WLAN and audio, but the graphics are not OK. Ubuntu offered to install&activate the current recommended Nvidia driver, but the machine hangs on shutdown/restart which prevents the installation of the proper Nvidia driver. I have to cycle the power to reboot.

I ran the Update Manager in the hope that the updates would fix the hang-up. At the end of the update-installation it asked to reboot - and got stuck just like before.

I see no obvious cause of the freeze and I don't know if it's caused by graphics problems or anything else. The only USB attachment is a mouse/keyboard; I don't have any external storage attached; and I don't have any programs running (the machine freezes even when doing restart right after login).

How can I determine what is causing the freeze?
How can I fix this?

I'm frankly rather disappointed because I bought this new machine in the hopes of getting the graphics to work, which failed miserably on my old machine, even though Ubuntu is supposed to be good with Nvidia. Being a fresh convert from Windows, I was hoping for a happier experience this time, so I'm very much looking forward to your suggestions!

... After posting this question, I see related questions in the right sidebar: this, this, and this. Don't know why these didn't show up while I composed by question. Those questions suggest some ACPI settings but I am not experienced enough to find/change those settings. I'll try the sudo shutdown -h now command when I get home and see if that works, then update this question. I did check the system BIOS but didn't see anything out of the ordinary.