This problem often occurs after switching from a single-core processor to a dual-core processor without reinstalling Windows XP. You replaced the motherboard and processor, but used the samehard disk drive with Windows XP Pro SP3 installed on it.
Note that if you have an OEM copy of Windows XP/Vista and you change the motherboard, you have to buy a new OEM copy of XP/Vista or a new licence, because an OEM copy can only be installed on one computer, and changing the motherboard is defined by Microsoft as changing the computer. You are only allowed to change the motherboard if you have to replace the existing motherboard with another motherboard of exactly the same make/model.
Product activation will detect a change of motherboard and refuse to activate and you will only be able to use the computer to activate Windows after 30 days. You can use a retail boxed copy of Windows XP/Vista as many times as you like as long as it is only installed on one computer at a time.
Apparently, Windows XP can detect the change in hardware and install multiprocessor support. However, it only works if you install a second processor on a motherboard that supports more than one processor, not if you change from a single-core to a dual-core processor. Windows XP has been known to detect the dual core and show two processors in the Processors section of Device
Manager, but if it does, it won't use the second core.
If you press the Ctrl+Alt+Del key combination the Task Manager presents itself. Select the Performance tab, click on the View menu and then select CPU History. (CPU is an acronym for central processing unit, which is the processor). If Windows is using both cores, there will be two options: One Graph Per CPU and One Graph, All CPUs.
There should also be separate graphs of processor performance side-by-side - the default setting. The bottom of the first window in CPU-Z shows the number of cores in use. The problem is in theHardware Abstraction Layer (HAL) used when Windows XP was first installed. Note that Windows Vista no longer uses HAL, which is probably why the dual-core processor was correctly recognised in Vista.
The Windows setup program normally detects the correct make/model of processor, but this depends on the computer's BIOS settings and can be affected if there are bugs in the BIOS. Device Manager shows which HAL you are using under Computer. The entries for dual- core processors is ACPI Multiprocessor or MPS Multiprocessor; the latter being best if your BIOS does not supportACPI power management properly, which all fairly recent motherboards do support, because the ACPI power standard first came out in 1996.
Other options are Standard PC (which your computer has, suggesting that ACPI was not loaded), Advanced Configuration and Power Interface (ACPI) PC, MPS Uniprocessor and ACPI Uniprocessor.
Microsoft recommends that the HAL should only be changed by reinstalling Windows XP, because the computer will not boot if anything goes wrong.
A repair installation of Windows XP requires a genuine Windows installation CD/DVD, which many computer manufacturers don't provide. A recovery system that is launched from a disc is usually used instead, which returns the computer to the state it was in when it left the factory (minus anything you have installed or created). In any case, Windows XP may still not detect a dual-core processor during a repair installation, making a complete reformat and clean installation necessary in order to upgrade to dual-core support.
Fortunately, there are ways around this state of affairs, which are not approved by Microsoft, because they can go wrong. Therefore, you should create a complete backup of your system before you begin, or at least make copies of all of your data files, images, emails, IE Favorites, etc.
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