Asus P5L-MX - Manual (Page 28)

1-16 Chapter 1: Product introduction
Important notice on installing Windows ® XP 32-bit version
If you install Windows ® XP 32-bit version Operating System (OS), the
limitation of this OS version is that it may reserve a certain amount of
memory space for system devices. We recommend that you install less
than 3 GB system memory if you would like to work under Windows ® XP
32-bit version OS. The excess memory installation will not cause any
usage problem, but it will not give users the benefit of manipulating this
excess memory space.
Visit the ASUS FAQ site for further explanation:
http://support.asus.com/faq/faq.aspx?SLanguage=en-us
Under General Search, make the selections as shown, then click Search.
Click the article titled “4GB memory installed but less memory size
detected.”
You also may check the URLs below for third party comments on this
issue:
http://dlsvr01.asus.com/pub/ASUS/mb/4GB_Rev1.pdf
http://www.intel.com/support/motherboards/server/sb/cs-016594.htm