My system is working perfectly but it freezes during shutdown/reboot/suspend/hibernate: All windows and the menu bar disappear but the desktop wallpaper remains. It doesn't even show the shutdown screen (the one with the animated dots) where I could hit ESC and watch the shutdown console text. The system is brand-new and fully updated using Update Manager.

1. How can I determine what is causing the freeze?
2. Is there a log I can investigate?
3. How can I fix this?

I see no obvious cause of the freeze. 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 shutdown right from the login screen).

What I've tried so far:

• Based on other questions (this, this, and this) that suggest some ACPI settings, I've tried sudo shutdown -h now to see whether the shutdown console text display offers any hints, but the system doesn't even get that far - it still freezes while the screen shown the desktop background image, without any toolbars. Only sudo shutdown --force works, but that's not a solution.
• Editing the grub menu to add acpi=off to the kernel didn't help. I guess there's not much point in trying the other (lesser) ACPI suggestions?
• Adding noapic to the grub entry had no discernible effect. Adding nolapic instead did something (I had removed the quiet option) - the system managed to continue further with the shutdown, right until the line Checking for running unattended-upgrades: which were the last characters on the screen.
• I've also checked the system BIOS, especially regarding power options, but didn't see anything out of the ordinary. Switching the BIOS standby setting from S3 to S1 didn't help. The standby setting can't be disabled, and there are no other ACPI-related settings AFAIK.
• BIOS reset didn't help. Not surprised; hadn't changed anything.
• I tried going to a virtual console (CtrlAltF1) as suggested by djeikyb and from there did a shutdown -h now and it froze there too, after this console output. I didn't try killing processes one at a time because I'm still too newbie to figure out how to do that.
• Booting with kernel 2.6.35.22 rather than 2.6.35.25 didn't help.
• Disabling the Nvidia drivers didn't help.
• Booting from Live CD (USB stick in fact) didn't help; it freezes the same way.
• Booting from Live CD, with acpi=off noapic nolapic didn't help either. Neither did just nolapic. So evidently this is not some custom setting in my install, but some sort of basic issue.
• MemTest competed in 1 hour without errors.