I could not find any BIOS updates to Acer M1641. Therefore I have to use "acpi=off pnpbios=off" option. There was still a problem. Reboot works. Shutdown does not work as it hangs.
Similar bugs were reported for Ubuntu 8.04.
The solution in the Launchpad bug report works.
This is certainly not an old PC and Vista 32bit SP1 works fine. However I tend to believe this is more an Acer Bios bug. Actually FreeBSD 7 hangs during startup. If I disabled ACPI in BIOS, FreeBSD 7 will complain about GPF (General Protection Fault) no matter which option I select for startup. If I enable ACPI in BIOS, it could not find the disk. So I could not run FreeBSD 7 at all with this new PC. So Ubuntu 8.04 is at least better than FreeBSD 7 in this aspect.
FreeBSD problem with Acer M1641 is reported here.
Notes on 10-April-2009:
The latest kernel does not need the options any more for Ubuntu 8.04.