[Karsh]

Код:

function Get_UserName: string;
var
  uname: pchar;
  unsiz: cardinal;
begin
  uname:= StrAlloc(255);
  unsiz:= 254;
  GetUserName(uname,unsiz);
  if (unsiz > 0) then
    Result:= string(uname) else
    Result:= 'n/a';
  StrDispose(uname);
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  ShowMessage(Get_UserName);
end;