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

http://pcsupport.about.com/od/fixtheproblem/ss/rconsole.htm

 

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.

 

http://www.geekstogo.com/forum/How-to-repair-Windows-XP-t138.html

 

then do a windows update and install the 90+ updates, as they are removed during the repair.

Thanks for your response.  Before I saw it I had already done a successful repair install of Win XP Pro SP2, fortunately with all data and settings intact.  I was trying to avoid this because of the potential risk of data loss, so I will keep your post as a reference in case it happens again in the future. 

Glad you repaired it, you are welcome.