most motherboards offer an option to leave a 1 meg memory hole, i believe it's between 15-16 meg ranges. this was originally designed for ISA compatibility, and even though ISA is long gone in most computers the bios's still offer to leave a memory hole.

i have a asus cusl2 based on the intel 815e chipset. i think the 815 chipset doesn't even support isa, but i still have that memory hole option in the bios.

anyways, check in your bios for this and disable it if it's there