Why Windows is showing Less Memory ?

I had Windows XP installed with one 256MB stick of RAM in it.I have installed one more 256MB RAM-stick which means I have double the speed of RAM but windows reports seeing only 192MB.

Why it is showing less memory?

I think there may be some mistake or error while installing a second memory-stick,the reason for not recognising second memory-stick

To add or upgrade computer memory,follow the installation guide below:
Turn the computer off.
Disconnect all of the cables and other connections into the computer, including the telephone line connection for the modem, the printer connection, the monitor connection, and everything else. Also unplug the power connector from the PC.
It's important that nothing external to the computer be connected before you start the upgrade so that nothing outside of the computer can provide any kind of electrical current to the computer while you are working on it. It's not getting electrocuted that's the concern, it's that some tiny electrical charge might come in at the wrong time and destroy a component.
It's not uncommon for a computer to draw a little bit of electrical current while it is plugged in, even when it has been powered off, so that's why it's important to unplug the power connector, too.

Each connection should only connect one way, so there's no need to remember how the connection was oriented before it was disconnected. But chances are you've got a nice little collection of connections into your computer, so you don't want to be looking at a confused pile of disconnected wires when you're done installing the memory.

It may be the issue of Shared memory
Shared memory means the onboard video has part of the system memory assigned to itself.
This will mean that windows won't report the full amount of ram. Depending on the chipset, you can usually adjust the amount of system memory it uses in the BIOS.

which motherboard are you having? there's a possiblility that one of the memory sticks is incompatible with that motherboard. Some motherboards can only use the lower density memory.