Author: Mike Shkolnik

How can I change the default printer in Windows?

Answer:

If you want to change the the default printer in Windows OS, you must change the 
device key in the Windows section of the WIN.INI. After that you must send a 
WM_WININICHANGE message: 

1   var
2     strIni: array[0..MAX_PATH] of Char;
3     arrWindows: array[0..64] of Char;
4   begin
5     GetWindowsDirectory(strIni, SizeOf(strIni));
6     StrCat(strIni, '\win.ini');
7     with TIniFile.Create(strIni) do
8     try
9       WriteString('windows', 'device', 'HP LaserJet 4 Plus,HPPCL5MS,LPT1:');
10    finally
11      Free;
12    end;
13  
14    StrCopy(arrWindows, 'windows');
15    SendMessage(HWND_BROADCAST, WM_WININICHANGE, 0, LongInt(@arrWindows));
16  end;