quite funny you ask about that, because I was just taking my inspiron 1100 apart to clean it to solve that ever-popular overheating issue.
I looked at the processor, and they're about the same size as the one in my desktop, and socket 478 as my desktop's. Heck, I can even put it in my desktop computer... I wouldn't risk it though, unless if you have excellent thermal solution and cleaned every speck of dust in there, because Pentium 4's ARE HOT.
Make sure the new processor matches the same socket number your lappy can hold.