You need to make sure that all of your computers are on the same subnet if you don't have a router. I would also make sure you have Client for Microsoft Networks installed as well as File and Printer Sharing for Microsoft Networks installed under your network connections properties so you can access shared resources.
Check the IP address on each system using ipconfig /all and compare the results of each one to make sure they have the similar IP addresses and identical subnet masks and gateway addresses.
If you are using DHCP make sure you have a DHCP server on the network giving out IP addresses. You can check this under the settings of your network connection as well by double clicking Internet Protocol (TCP/IP).
Also make sure you have something shared with the appropriate permissions so you have something to browse for and connect to on the network.
Depending on how you have your network setup you may need to adjust the user permissions or make accounts for the other users so they are able to access your shares.