Firstly you must set one or both the machines to share files. You must also have admin privilages.
Here's what to do. click on start and go to the control panel. Next double click this sequence. "administrative tools", then Sevices". In the box that appears go to "Computer Brouser" right click this and choose start. Scroll down the page now and look for the entry "Server". Right click on this and select start.
By right clicking these two entries and choosing properties there is a tag which will allow you to set the PC to automaticaly start these services if you wish to do so. Thereby avoiding the necessity of starting these every time you re boot the computer.
For now you've done enough to allow sharing on the computer. Test it if you wish by right clicking on any folder in the system. You will see there is an extra option called share and security.
On the desctop PC:
Lets say you have a folder called "MyFolder". Right click this and choose the share tab if not already highlighted. Next put a in the box "Share this folder on the network". As soon as you do this you will see the name of the folder "MyFolder" in the text box. You can give this folder any shared name you desire but for the purposes of this demonstration leave it as it is. Now click on Apply and OK to close the box. Now close all remainingopen boxes.
Now go to the laptop. Assuming they are both attached to the network. You'll need to know the name of your desctop PC. Lets say it's called MyComputer. Here's what to do.
For this demo I've called you desktop PC "MyComputer" and The now shared file on it "MyFolder". Click on start and select "Run". in the window that appesrs type exactly this, \\MyComputer\Myfolder. You should now see the shared folder on the desktok PC open on your laptop.