Is the system you want to access able to browse the network and connect to other systems?
In the Network and Sharing Center, is that system in a Private or Public network?
Also, Under Advanced sharing settings check the following:
1. Under Private Profile, under HomeGroup connections, set to the Recommended setting.
2. Under All Networks, make sure that Password Protected Sharing is turned off, unless you actually need this. (Which in a workgroup setup you most likely do not).
Of course, the most obvious question (To me) is: Have you shared something? If so, did you include the Everyone user?