Finally my cousin noticed these notches (better photos at the bottom):
And voila, you can remove the keyboard without bothering with any screws...
Now here we obviously get access to the CPU by unscrewing the screwes marked with red arrows. (The red circles show the location of the notches.)
To actually remove the CPU you have to turn the marked screw counterclockwise by about 180 degree. (My Mobile Pentium III handbook states that this kind of socket is called FoxConn 495L)