Alright, XP does make it a bit easier. Do you have XP Pro? Cause pro may have some more steps to take.
Right-click My Computer on the host (PC with printer) and then click Properties. Next click the Computer Name tab. Look at the Workgroup name, write it down. Also make note of the "full computer name". Go to this same place on the client PC (other one) and if needed, click the "Change" button to put it on the same workgroup.
Next open My Computer. Then click Tools-Folder Options. On the host PC. Go to the View tab and scroll to the bottom of the list and see if there is a check mark next to an item called "Use simple file sharing". If so, uncheck it. May not be needed, but not sure.
Now go into Control Panel-Printers on the host. Right-click the printer and choose the sharing option. Check that the share name is nice and small and that the appropriate permissions are set to allow access.
During these operations, you may be required to restart, that's fine.
If you have done all these and still can't connect to the share, then it is likely a firewall problem. Since you have a router (which should also be a firewall), make sure neither computer has a software firewall running. Check your Antivirus, if it's some versions of McAfee or Norton or Panda, it may have a firewall function, turn it off.
To check the Windows firewall, go into Control Panel and click Security Center. There should be a link to firewall settings on the bottom, turn that OFF on both PCs as well.
Then try to connect to the shared printer again.
I personally think it's a firewall problem, as XP is normal extremely easy for printer sharing. In fact, if certain settings are in place, XP will automatically find and install shared printers, without you even having to do anything.
Good luck