Hello Greg,

you might use the MacAddress of the network card... this is unique and specific to a machine.

in .NET

                Dim query As System.Management.ManagementObjectSearcher = New System.Management.ManagementObjectSearcher("SELECT * FROM Win32_NetworkAdapterConfiguration where MacAddress is not null and IPEnabled = true")
                Dim queryCollection As System.Management.ManagementObjectCollection = query.Get()
                Dim mo As System.Management.ManagementObject

                For Each mo In queryCollection
                    Return mo("MacAddress").ToString
                    Exit For
                Next

Hope that helps
Alex