Thats impossible. Something must be wrong with your friend's keyboard or pc.
PS\2 sockets only transmit data through 1 pin.
The other three are clock, power and ground.
Yes, as you can see only 4 or the 6 pins are utilized.
They other pins become utilized when you diasy chain devices such as mouse->keyboard.
How do you explain CTRL+ALT+DEL.
Or crouching and moving forward with strafe.
Each key has a set of binary codes which are then converted to a hex value.
Lets say you hold a key down.
The hold key code will be added to the entered key and transmitted and decoded by the pc.
If this were compared to wireless protocols, than it would be close to TDMA.
USB offers a full duplex of data with 2 data connectors instead of transmitting everything on one wire....therefore there is also faster data transfer and better response....just like CDMA.
Also, its PNP, and can support non keyboard devices like lcds(i've seen a keyboard with an lcd on it.)
To use a USB keyboard...you will need to configure your board to handle a USB keyboard or you computer may no boot.
USB mice offer similar pros and cons