I am going to assume both laptop and pc are Windows XP (home or Pro) and both are able to be connected to a wired network (have a network port on each).
1. Purchase a cat5 cross patch cable; this is not a cat5 straight patch cable. It should be long enough to connect the two pcs.
2. Set up a static subnet for your use. I will define the desk top as default gateway in this application.
2.a. On the desktop, right click on network connections. Select your wired network connection, right click on it and click on properties. Highlight Internet Connections (TCP/IP) in the pop up window and then click on the properties button just below the Internet Connections (TCP/IP) highlighted selecton. This gives you a pop up. Change from Obtain IP adress automatically to Use the following IP address and enter this information: IP address 192.168.1.10, subnet mask 255.255.255.0, default gateway 192.168.1.10. Below that select Use the following DNS and enter 192.168.1.10. Save this and exit out.
2.b On laptop set up its IP address using the same proceedure. There is one and only one difference (the ip address itself). Change from Obtain IP adress automatically to Use the following IP address and enter this information: IP address 192.168.1.11, subnet mask 255.255.255.0, default gateway 192.168.1.10. Below that select Use the following DNS and enter 192.168.1.10. Save this and exit out.
3. Plug in the cross cable into both units; it makes no difference which you open first. Make sure all firewalls on both machines are off (done in control panel for Windows firewall, done in the firewall software if you have a different firewall).
4. At desk top pc, open command prompt (start, run - enter cmd in the box and hit enter). At the > prompt, type in "ping 192.168.1.10" (not including quotes) and hit enter. You will see 4 positive replies at the end of the response. This means the desktop has seen itself. Now in command prompt, type in "ipconfig 192.168.1.11" without quotes and hit enter. You should see four successful replies just like you did when you used "ping 192.168.1.10". If you are not successful, you made an error in assigning IP addresses in step # 2 and you need to check both out to find error. If you are successful, you now have a link between the 2 pcs.
5. At laptop, open command prompt and type in "ping 192.168.1.10" and hit enter. You should see 4 successful replies - meaning lap top is connected to desktop.
6. Now if you want to swap files between the two machines, remember to permit sharing in Windows Explorer. Open Windows Explorer by right clicking START and clicking on Explore. Highlight the folder you wish to permit sharing and follow the sharing prompts. This permits the other pc to see the folder or folders you wish to share.
7. After you have completed your swap of files, you may wish to return the network settings to obtain IP automatically.