Google is your friend (third hit)
"Here is the definitive fix, and yes, device manager has been known to be wrong when it reports two CPU's on Hyperthreaded systems. If the multiproc kernel and hal aren't on there and matched, then neither the OS nor any app is really going to see or take advantage of Hyperthreading. If msinfo32.exe or systeminfo.exe reports two processors, you don't have this problem.
1) Play safe - leave your self a couple of fail-back routes.
Make a new (or update your current) repair disk (Program, Accessories, System Tools, Backup)
Insert your original XP Pro CD (if you're using XP Home, give up now this isn't going to work) and run <cd>\i386\winnt32.exe /cmdcons. Make sure that you are connected to the internet so setup's auto-update feature can work (you have probably long since installed SP 1 and numerous hot-fixes). This will install the command prompt recovery console as a boot option.
Hopefully you won't need this if you carefully follow the steps below, but it is always good to have it there anyway (you did make that repair disk too didn't you?). I didn't need to use this and can't guarantee it will work though. It probably still needs a matched hal/kernel. Forget all this if your boot partition is Fat/Fat32 (and < 2GB), and have a Win9x boot floppy handy, or format a new one and check the "Create an MS-DOS startup disk" when you do. The second option is the preferred since it should allow you to see up to 4GB Fat/Fat32 partitions.
Test your recovery console to make sure you can boot to the console. If, like me, your boot adapter driver is not in-box make sure you have a driver floppy handy when the "Press F6 to install SCSI/RAID drivers" prompt rolls around or you will bugcheck with a "7B". If you can boot to the console, good, go on to step two. Don't bother with setting a system restore point, this is too low-level.
2) Go to %windir%\system32 and make copies of hal.dll and ntoskrnl.exe (hal.dll.sav ntoskrnl.exe.sav or something). I usually put the copies in two places just to be safe when mucking around with the kernel.
If you are not running XP Gold (build 2600 - winver will tell you if you aren't sure) find your service pack files, (should be in %windir%\ServicePackFiles\i386. If you are running Gold, you can get the same files in compressed form off the CD\i386 directory and expand them to a temp directory.
Now copy halmacpi.dll to %windir%\system32\hal.dll and ntkrnlmp.exe to %windir%\system32\ntoskrnl.exe. CAREFUL with the names, they must be exactly as stated above. These are the ACPI compliant multi-proc components. Don't worry about ntkrnmpa or others, they are for PAE and APIC based systems. The p4c800 is neither, and neither are 90% of any other desktop/workstation boards, so this will work for 90% of you out there.
If your board supports more than 4GB of memory, you have a "PAE" system. If so I am jealous, but this will still likely work, as there are even less APIC boards out there. If you breathe this rarified air, the kernel/hal you are loading will essentially throw away any memory over 4GB. Email me if this is the case and I'll discuss the different hal/kernel options, or better yet, go to the OSR website and search for an extensive discussion of what is what and how to tell what you are already running.
Reboot and pray your hardware sacrifices to the Gods of PNP over the years have been sufficient. On reboot (if you don't bugcheck with "Ntoskrnl is missing or corrupt..."), the Hardware Device wizard will run and detect a "new" ACPI system and then require a reboot. If you got this far all is well with the world, go ahead and reboot. On the second boot you will likely have a long blank screen period where nothing appears to be happening - be patient, when this doesn't work a bugcheck will always occur very quickly.
Run devmgmt.msc from the Run box and go to Computer. Do you see "ACPI Multiprocessor PC"? Good. Run msinfo32.ex from the Run box, see two processors? Good. Double check by running systeminfo.exe from a command prompt. Again, see two processors? Great, you are good to go. "
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