Memory issues

Hello,

I'm not good with computers so please help me. i recently got a 1gb ram for my pc, but it registers only 512mb. Even in the bios it just registers 512mb. So i took the ram and plugged it in my other pc ( which is of the same make and has the same configurations) and magically it registers 1gb. i took it and tried it in my friend's pc which is of a different make and it works correctly. im at my wits end now.i really dont know whats wrong and i cant figure it out. I tried using different rams ( 256 mb, 512 mb and they register correctly)but somehow this particular 1gb ram has decided to make my life hell.....please help me. Im using windows xp professional with sp3 and everything else is up to date.