You have 2 devices - main computer and "clever " keyboard.
Between them will be 2 wires - SDA,SCL.
On main computer You can specify (in source code for example)
any address, on keyboard You must define same address.
Take care to not use address of other i2c chips on Your board.
My choice is 0xAE .
Intersting page:
http://members.fortunecity.com/ultravinn...l#extended

Now You have to learn I2C , to write master I2C routine for main and slave routine for keyboard controller, to solder board
with keyboard and slave controller.

In Your situation i will:
- learn appnote from Philips (about PS2 keyboard)
- copy/paste/integrate/compile code from this apnote (code is good)
- make 4 simple wire conections between any PC keyboard and
main xx89xx51xx.
- At later stage, if You need embedded keyboard, You can
programm slave chip to work as "standard" controller for PC keyboard.

I2C is good , PS2 is practical.

regards