256 MB ram missing

Hi, I installed a RAM of 256 MB. I alreay have 1 GB RAM. And now 256 MB. Total 1.25 GB I have in my c. In my computer properties, it is showing as 1.25 GB RAM installed. But i am seeing only 1 GB. 48.7and 51.3. Which means 256 MB is missing.
Will someone give solution to my problem??????????