|Try a restore point first. If that doesn't work, you might have to use the XP installation disk to do a repair. If you do have to do a repair it would be a good idea to clone the drive to an external drive with some sort of cloning software or a boot cd or dvd with such software on it. There are several iso's on the net that will do a clone. I recently used Hiren's Boot CD 11.1.|
After the repair you will have to reinstall all of the update fixes.
Another way to fix this might be to use another computer to scan your XP installation disk cab files with Winzip or another cab capable archiver for the acpi.sys file and extract it to a usb flash drive or a usb external HD. Then boot up with Hiren's Boot CD and select the mini XP. Plug in the usb device and copy over the file you extracted from the cab to the appropriate system folder on your PC's HD. I used to do this when one of my system files got corrupted back in the days of Win 95.