XP 64 bit not showing all RAM

I have a dual boot system running both XP and Ubuntu 64 bit.

I have 8 GB of DDR2 RAM installed.

XP shows 2.75 GB of RAM in System, while Ubuntu shows 7.8 GB.

XP was showing almost 8 GB until just recently.

I tried resetting the BIOS and that did not work.  It was working fine on
the default BIOS settings recently.

Is there anything within the OS that needs to be tweaked in order for it to
recognize all the RAM again?

My hardware:

Intel Pentium E5200 Dual Core @ 2.50 GHz
XFX nForce 780i Motherboard
8 GB DDR2 RAM (4 X 2 GB) Dual Channel.
NVIDIA GeForce 9800 GT 1 GB video card.

Thank you

See if your BIOS has a setting to enable Memory Remapping, or some othername

See if your BIOS has a setting to enable Memory Remapping, or some other
name that sounds like it would be the same thing. Enabling this setting
tells the bios to remap peripheral ram above the physical ram. Without that
enabled, peripheral ram defaults to the top of the 4GB range, limiting
available ram to whatever is left below that. In your case the 1GB video
ram, plus another .25GB, seems to be sitting right below the 4GB range and
that is why you are seeing 2.75GB.