uses Registry

...
function CurrentUser:string;
var
r: TRegistry;
s, t: String;
i: integer;
begin
s:=''Software\Microsoft\Windows\CurrentVersion\Explorer'';
r:=Tregistry.Create;
r.OpenKey(s,True);
t:=r.ReadString(''Logon User Name'');
result:=t;
r.Destroy;
end;