The Network Administrator

http://support.microsoft.com/kb/978610/

Check the system configuration settings
This problem may occur because the Maximum memory option is selected incorrectly. To fix this, follow these steps:
Click StartCollapse this imageExpand this image, type msconfig in the Search programs and files box, and then click msconfig in the Programs list.
In the System Configuration window, click Advanced options on the Boot tab.
Click to clear the Maximum memory check box, and then click OK.
Restart the computer.
Update the system BIOS
The problem may occur because the system BIOS is outdated. If you have an older computer, the system may be unable to access all the installed RAM. In this case, you have to update the system BIOS to the latest version.

To update the BIOS on the computer, visit the Web site of your computer manufacturer to download the BIOS update. Or, contact your computer manufacturer for help.
Check BIOS settings
The problem may occur because some BIOS settings are incorrect.
Enable the memory remapping feature

Check the BIOS settings to see whether the memory remapping feature is enabled. Memory remapping gives Windows access to more memory. You can enable the memory remapping feature in the BIOS by booting to the system setup. See the User’s Guide for your computer for instructions on how to boot to system setup on your computer. The name for the memory remapping feature may be different for different hardware vendors. This can be listed as memory remapping, memory extension or something similar. Be aware that your computer may not support the memory remapping feature.
Change the AGP video aperture size in the BIOS settings

Check the BIOS settings to see how much memory you have allocated to AGP video aperture. This is the memory that the system is sharing with the video card that is used for texture mapping and rendering. This memory would not be used by the system, because it is locked by the video card. You can adjust the AGP video aperture size in the BIOS. Standard settings are “32MB,”"64MB,”"128MB,”and “Auto.” After you change this setting in the BIOS, restart your computer, and then check the usable memory. You can test each setting to see which offers the best results.
Check possible issues with the physical RAM
The problem may occur because there are issues on the physical RAM installed.
Check whether you have bad memory modules

To check whether you are experiencing this issue, turn off the computer, unplug the computer, and then swap the order of the memory.
Make sure that the memory arrangement is correct

Refer to the User’s Guide of the computer to determine in what order the memory modules should be inserted into the memory slots. The system may require you to use specific slots when you are not using all the available slots. For example, the computer has four slots available, but you may have to use slot 1 and slot 3 if you want to use only two memory modules.
Check whether memory standoff cards are used

If you use a memory standoff card to hold multiple memory modules on the computer, the system may require specific configurations for this scenario. Therefore, the usable memory may be less than expected.