|I have already SOLVED the problem !!!!!!!!!!!|
I tried to check each option of bios again using F1 key for help. There is a "OS Installation Mode" option whose value should be either ON or OFF(default). When I pressed F1 key it gave me a message like this.........
"if this is on then total memory will be 256mb of total memory for operating system but if this is off(default) then operating system will get the full system memory.
It was "ON", so operating system memory was 256mb only whereas total memory is 512mb. But, now I set that "OFF" and OS have total 512mb as shown in my compouter properties.
Thanks for your co-operation.