I recently corrected the same problem on a pc which indicated in bios it was
acpi compliant and had also had an upgrade from Windows 98 to XP. The
upgrade either had an incorrect response at some point (1st timer performed
the upgrade) or XP missed detecting acpi compliancy. I followed the link
suggested by Rich Barry (kb article) and worked through the suggestions
there. None corrected the problem and I ended up doing an XP repair install
(if you google "how to" you may be told the acpi option is shown at the raid
driver load prompt - I couldn't find any prompts there) and just followed
normal prompts - all fixed. If you end up doing this make sure you go into
the actual repair install, not the repair console. Also check afterwards re
updates and patches for Windows which may need reloading.
Cheers,
Peter.