acpi.sys is related to the multiprocessor PC name.
It is in the i386 folder of the XP CD, but it is compressed, and is labeled acpi.sy_
http://support.microsoft.com/kb/888017/en-us use method 4 to expand it from CD to your hard drive that has the corrupted acpi.sys
How to start the "recovery" console
so the command would look something like this, substitute drive letters with the correct ones, they may be different.
expand D:\i386\acpi.sy_ C:\windows\system32\drivers hit enter, be sure to include spaces between expand and D, between _ and C
D:= your optical drive the XP CD is in
C:= your hard drive that will not boot.
When you first get to the recovery console command prompt use the command map and hit enter, it should list all drives connected and what the drive letters are.
If this fails to give results, do a XP repair as described in this article.
then do a windows update and install the 90+ updates, as they are removed during the repair.