Can you try this one:
Select query.Domain, query.Username, Count(query.Computername)
From (Select Distinct tblCPlogoninfo.Computername, tblCPlogoninfo.Domain,
tblCPlogoninfo.Username
From tblCPlogoninfo
Group By All tblCPlogoninfo.Computername, tblCPlogoninfo.Domain,
tblCPlogoninfo.Username) query
Group By query.Domain, query.Username
Order By Count(query.Computername) Desc