When installing Ubuntu 10.04 under Wubi on my ThinkPad T43, on the first reboot I got this message:
The installer encountered an unrecoverable error and will now reboot.
That was it. No error message, no way to continue.
After a reboot, the same message would pop back up again.
I saw one mention of a work-around here on page 2 of this thread:
The fix was to hit Esc before the GUI loads and select "ACPI workarounds".
This appears to be fixed (on the T43) as of Ubuntu 10.04.1 and 10.10.