1) Both machines be assigned the same workgroup name.
2) Any firewalls set to allow file and printer sharing.
3) Some folder set to be shared.
4) The printer set to be shared.
Here's a more formal guide:
I'm trying to network two computers and a printer at my home using a Microsoft MN-100 Router. Both computers are HP computers, running Windows XP Home.One of them is an (HP Pavilion a350n, and the other is an HP Pavilion a712n) Right now, both computers are connected to the internet, but I can't enable filesharing or Print Sharing. My printer is connected to the a350n, but when I installed the router the a712n was consider the main computer. Please help!