I would first check to make sure they are in the same workgroup by going to the system properties of both and then look at the Computer Name section. Plus make sure you have something shared on the computer so the share will show up when you browse for it.
Next you should check the IP configuration by opening a command prompt and typing in ipconfig /all and compare the results for the IP addresses and subnet masks. The IP addresses should be similar except for the last group of numbers such as 192.168.0.2 and 192.168.0.3. The subnet masks should be the same. Once you get both IP addresses you can ping one from the other. For example on the 192.168.0.2 computer type in ping 192.168.0.3 from the command prompt and see if you get a reply.
If you have any firewall software such as Norton Internet Security there is a good chance that it is blocking the connection. Check the settings or disable it to test it. You might also disable the Windows firewall temporarily to see if that's causing the problem.
If everything checks out, do a search by computer name or IP address from Windows Explorer. From the address bar type in either the computer name or IP address and see if it comes up in a search. It should be in this format: \\computername or \\IP address.