Windows XP Pro & Windows Server 2003 operating systems are able to see more than one processor. Windows XP Pro is also designed to take full advantage of the Xeon’s Hyper-Threading Technology, if is enabled in the CPU BIOS parameter.

Windows 2000 is only able to see one processor and does not take advantage of HT technology. You will need to upgrade to Windows XP Pro.

Thanks chipstone
but it means if i work under windows 2000 i use only one processor ??? or two but i'm able to see just one. I tried to run sisoft sandra 2004 and it says that i have just one processor installed.

Add Support for Multiple Processors in Windows 2000
Installing Support for Multiple CPUs
1.Click Start, point to Settings, point to Control Panel, and then click System.
2.Click the Hardware tab, and then click Device Manager.
3.Double-click to expand the Computer branch. Note the type of support that you currently have.
4.Double-click the computer type that is listed under the Computer branch, click the Drivers tab, click Update Driver, and then click Next.
5.Click Display a list of known drivers for this device, and then click Show all hardware of this device class.
6.Click the appropriate computer type (See Below), click Next, and then click Finish.

Select the following computer type from the list:

ACPI MultiProc (Used for a multiple-processor ACPI computer)

I don’t believe that you will see the two processors in device manager, but if you change the computer type to the above the system will utilize the dual processors.

I would strongly suggest upgrading to Windows XP Pro. The HT will give your applications a very noticeable performance increase.

P.S. This is additional info from Microsoft Website on the change to make Windows 2000 recognize dual processors:

SUMMARY
Windows 2000 provides support for single or multiple Central Processing Units (CPU); however, if you originally installed Windows 2000 on a computer with a single CPU, the hardware abstraction layer (HAL) on your computer must be updated for your computer to recognize and use multiple CPUs. This article describes how to add support for multiple processors in Windows 2000. In Microsoft Windows NT 4.0, you can use the Uptomp.exe tool to add support for multiple CPUs. The Uptomp.exe tool is not used in Windows 2000, instead, you can use Device Manager to make these changes. Note that before you change the computer type, contact your computer manufacturer to determine if there is a vendor-specific HAL that you should use instead of the standard HAL that is included in Windows 2000.

Thanks a lot Chipstone
Now i have got 2 processors runing. Nice.

Thanks again.