Автор: http://www.swissdelphicenter.ch

 function GetCurrentUserName: string;
 const
   cnMaxUserNameLen = 254;
 var
   sUserName: string;
   dwUserNameLen: DWORD;
 begin
   dwUserNameLen := cnMaxUserNameLen - 1;
   SetLength(sUserName, cnMaxUserNameLen);
   GetUserName(PChar(sUserName), dwUserNameLen);
   SetLength(sUserName, dwUserNameLen);
   Result := sUserName;
 end;

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