Some ram missing...

Dear all,
Have got a weird missing ram problem.... recently, I bought an extra A-DATA 1Gb ram from my friend so I have got totally 2G DDR2(1G X 1G) ram installed but somehow the BIOS and Windows XP Sp3 only recognize 1.5G of ram, when I check with CPU-Z, it shows the correct amount of 2Gb ram, I have tried using only 1 slot and both ram are detected correctly(I just cant use them both?!) is there anything else I can try to get the missing ram back?

my system spec.:
MB: ECS P4M800PRO-M3 with latest BIOS
CPU: Celeron D 3.2G
RAM: 2GB ram total(only 1.5G recognized)
OS: Windows XP SP3