You need a BIOS Upgrade for your motherboard to accept the new CPU.
And maybe the BIOS doesn't accept the P4M.
ANd P4M is recognized to be not very good in term of battery life ...
I own an Acer travelmate 2201LC laptop with a Intel Celeron 2.6 GHz CPU. The batterylife wasnt too good so I tried to upgrade the cpu to a less power hungry model (the celeron eats up 70-something watts). First I got a Pentium M thinking that when it just says 478-pins it must be the same. So of course I found out that it doesnt physically fit in the socket (no fair, couldnt they have called it something else then?). I then found out that a pentium 4 m would fit in the socket, and now I finally got a hold of a 1.8 GHz Mobile Intel Pentium 4 M. When I installed it I saw that it doesnt have a thermal plate like the celeron making it slightly thinner. I put on some ekstra thermal grease to make up for the gap (not ideal i know) but when I turned the laptop on it just gave a short beep and turned off again.
If you dont want to read a long story skip to here:
-Is a laptop running a Celeron CPU not compatible with a Pentium 4 M CPU?