Stole this from an older thread:
Check to see if selective start is selected.
If yes, go to the boot tab.
Click "Advanced Options".
Deselect the maximum memory option if it is selected.
Also, you can optionally select the number of processors to increase the boot speed.
Click on apply to apply the new settings.
Reboot the computer. Once the computer is rebooted, right click on "Computer" on your desktop and select properties. Installed memory should now show just the memory installed rather than the 2 components (installed and usable).
Hope that helps