memory upgrade

I run winME on a Cel400mhz, when i added 256mb the device manager showed 128mb, and when i replaced it with 128mb the indication was 64mb
When i tested the above dimms with a new winXP PC the indications were correct(256mb/128mb)
Why the old pc shows only the half of the memory? Is there anything wrong with memory slots of the motherboard? Any suggestions would much appriciated. Thanks