A CPU has socket numbers the most recent socket is 775 for intel, this is the positioning of the pins and also means what type of CPU you can place in the mother board.
Lets say you can't put a p4 CPU in a 775 socket on a mother board. So thats why you will not be able to place your laptop CPU into the desktop as they are incompatible, the one has p3 (pentuim 3) the other pentium 4.
So the mother board and CPU are not compatible.
I believe you should upgrade to a cheap computer such as amd which is alot cheaper than intel so you can get a ok pc for little money.
Hope this helped.
Vlad
