這段程式已使用多年都沒問題
SendMessage(HWND_BROADCAST, WM_WININICHANGE, 0, Longint(@ResStr));
procedure SetDefaultPrinter1(NewDefPrinter: string);
var
ResStr: array[0..255] of Char;
begin
StrPCopy(ResStr, NewdefPrinter);
WriteProfileString('windows', 'device', ResStr);
StrCopy(ResStr, 'windows');
SendMessage(HWND_BROADCAST, WM_WININICHANGE, 0, Longint(@ResStr));
end;

編輯記錄

用以下程式測試結果, 
可改變印表機設定, 但是
SendMessage(HWND_BROADCAST, WM_WININICHANGE, 0, Longint(@ResStr));
這行指令好像變成不歸路,無法執行下一行指令,
也就是說
SetDefaultPrinter(Combobox1.Text);指令執行後,程式就結束了.
以下指令無法執行
ShowMessage('Set Printer ok!');
不知問題何在?

unit Unit1;
interface
uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, IniFiles,UrlMon, StdCtrls, Gauges, ComCtrls, ExtCtrls;
type
TfrmMain = class(TForm)
Panel1: TPanel;
OpenDialog1: TOpenDialog;
Label1: TLabel;
ComboBox1: TComboBox;
Button2: TButton;
procedure FormCreate(Sender: TObject);
procedure Button2Click(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
end;
var
frmMain: TfrmMain;
implementation
{$R *.dfm}
uses
Printers;
function GetDefaultPrinter: string;
var
ResStr: array[0..255] of Char;
begin
GetProfileString('Windows', 'device', '', ResStr, 255);
Result := StrPas(ResStr);
end;
procedure SetDefaultPrinter(NewDefPrinter: string);
var
ResStr: array[0..255] of Char;
begin
StrPCopy(ResStr, NewdefPrinter);
WriteProfileString('windows', 'device', ResStr);
StrCopy(ResStr, 'windows');
SendMessage(HWND_BROADCAST, WM_WININICHANGE, 0, Longint(@ResStr));
end;

procedure TfrmMain.FormCreate(Sender: TObject);
begin
//Fill the combobox with all available printers
Combobox1.Items.Clear;
Combobox1.Items.AddStrings(Printer.Printers);
// Get the Default Printer:
label1.Caption := Format('Default Printer: %s',[GetDefaultPrinter]);
end;
procedure TfrmMain.Button2Click(Sender: TObject);
begin
//Set the selected printer in the combobox as default printer
if Combobox1.Text <> '' then
begin
SetDefaultPrinter(Combobox1.Text);
ShowMessage('Set Printer ok!');
end
else
ShowMessage('Choose a Printer first !');
end;

已解決.
電腦重新整理就ok了.

請問電腦重新整理是怎樣整理?, 我也遇到完全一樣的問題