If you dont have a router, and just have a CAT5 cable
you can, connect one cable to the laptop NIC and the other to the desktop NIC, if your NICs are newer you will not need a special type of ethernet cable called a crossover cable, if they are older you will need it.
This is a website on how to make a cross over cable and a standard ethernet cable.
Standard ethernet and crossover http://www.ertyu.org/steven_nikkel/ethernetcables.html
IF you have a router, plug both Ethernet cables into the NICs, right click on MY COMPUTER,go to properties, then look under COMPUTER NAME. while you are there put both computers in the same WORKGROUP =, the default in XP is MSHOME, that will be fine.
open windows explorer, clear the address bar, type \\PCNAME (PCNAME = whatever the name of your PC is) you should then be able to access the PC.
unless it is not setup that way, if that is the case, the easiest solution is to Run the network identification wizard from my network places, Just let me know if it comes to that and i will help you in further detail.
good luck man