I'll assume you have WinXP.
First make sure you both belong to the same workgroup. The easy way to do this is press and hold the Windows Key and then hit the PAUSE/BREAK key. Click the tab that says Computer Name. If both computers don't have the same workgroup name, click the Change button. Please also make sure that each Computer Name is different.
Reboot...
Now it's time to share the printer. Open Control Panel...Printers and Other Hardware...Printers and Faxes. Right click on the printer connected to your computer and choose Sharing. Click the "I understand the risks...". Click Share This Printer, Then Apply. Close all open windows.
Reboot
Go to the other computer and reboot it if it was still on while you were doing the other printer. When you are ready, go to Control Panel...Printers and Other Hardware...Printers and Faxes. Under Printer Tasks on the left, Click Add A Printer.
Hit Next. Click the Network Printer then Next. Choose Browse for a Printer then Next. If everything went properly, you should see the other computer and printer. Choose the printer and complete the wizard. When you are done, follow the same instruction to share that computer's printer with the other computer.