First thing, turn off the security software temporarily to troubleshoot.
Check to make sure port 135 and 139 are not blocked in any software firewall you may be running, or in the router itself (used for file sharing). The computers should be in the same workgroup (right-click ‘my computer’, click on ‘computer name’ tab).
Try pinging one computer from the other to see if it responds on the network. Open the start menu, click the ‘run’ button, and type ‘cmd’. When the command prompt window opens, type ‘ping XXX.XXX.XXX’, replacing the X’s with the other computer’s IP address (which you can find on the other computer typing ‘ipconfig’.