wrong specificatin at sytem properties in windows XP

I am using a latest kind of laptop which was Preloaded with windows home , due to its limitation i changed it to windows professional.but now the problem is that when i am taking system properties it is showing only 256 MB RAM where it should showing 512 MB of RAM which is present there.I have checked the with the display memory its just 64 MB.At bios it is showing 512 MB .At system Information at accessories -----system information it is showing 512 MB RAm .iwant to now why at system properties it is showing only 256 MB RAM.