Windows XP Pro SP2 - 4GB RAM but Windows only shows 3 GB

Hi,

I have 2 Dell laptops with same hardawre configuration, 4 GB RAM and Windows
XP Pro SP2.
They have been bought in an interval of one month so BIOS is not exactly at
the same version.
On the oldest one, Windows correctly displays 4GB, for instance in task
manager when the most recent one only displays 3GB.
I precise hat winmsd displays that my RAM is 4 GB in both laptops.

Sorry if it is a FAQ but I didn't find the answer when I searched in this
forum.
Thank you for reading this message and thanks to those that can help to
solve this problem.

Michel