When you open the Start Button, right-click on Computer and select Properties. A little ways down the window that pops up you should see a section called Computer name, domain, and workgroup settings.
To the right of Computer Name you should see a Change Settings link. If you click that you should get another pop-up window titled System Properties. It should be on the Computer Name tab, but if not select the Computer Name tab. The bottom button should say "Change". Click it.
You will then be able to see the workgroup or domain that you are connected to, as well as your computer name. Two things to verify here, make sure that the Computer name for both of your systems is different, and that the Workgroup for both of the systems is the same.
Alternatively, you can use the Network ID button to utilize the built in wizard to help you create and connect to the the workgroup.