Inno Setup: How to automatically uninstall previous installed version?

I have used the following. I'm not sure it's the simplest way to do it but it works.

This uses {#emit SetupSetting("AppId")} which relies on the Inno Setup Preprocessor. If you don't use that, cut-and-paste your App ID in directly.

[Code]

{ ///////////////////////////////////////////////////////////////////// }
function GetUninstallString(): String;
var
  sUnInstPath: String;
  sUnInstallString: String;
begin
  sUnInstPath := ExpandConstant('Software\Microsoft\Windows\CurrentVersion\Uninstall\{#emit SetupSetting("AppId")}_is1');
  sUnInstallString := '';
  if not RegQueryStringValue(HKLM, sUnInstPath, 'UninstallString', sUnInstallString) then
    RegQueryStringValue(HKCU, sUnInstPath, 'UninstallString', sUnInstallString);
  Result := sUnInstallString;
end;


{ ///////////////////////////////////////////////////////////////////// }
function IsUpgrade(): Boolean;
begin
  Result := (GetUninstallString() <> '');
end;


{ ///////////////////////////////////////////////////////////////////// }
function UnInstallOldVersion(): Integer;
var
  sUnInstallString: String;
  iResultCode: Integer;
begin
{ Return Values: }
{ 1 - uninstall string is empty }
{ 2 - error executing the UnInstallString }
{ 3 - successfully executed the UnInstallString }

  { default return value }
  Result := 0;

  { get the uninstall string of the old app }
  sUnInstallString := GetUninstallString();
  if sUnInstallString <> '' then begin
    sUnInstallString := RemoveQuotes(sUnInstallString);
    if Exec(sUnInstallString, '/SILENT /NORESTART /SUPPRESSMSGBOXES','', SW_HIDE, ewWaitUntilTerminated, iResultCode) then
      Result := 3
    else
      Result := 2;
  end else
    Result := 1;
end;

{ ///////////////////////////////////////////////////////////////////// }
procedure CurStepChanged(CurStep: TSetupStep);
begin
  if (CurStep=ssInstall) then
  begin
    if (IsUpgrade()) then
    begin
      UnInstallOldVersion();
    end;
  end;
end;

Alternatives

See also this blog post "Inno Setup Script Sample for Version Comparison" which goes one step further, and reads the version number of any previously installed version, and compares that version number with that of the current installation package.

You should be able to read the uninstall string from the registry, given the AppId (i.e. the value you used for AppID in the [Setup]-section). It could be found under Software\Microsoft\Windows\CurrentVersion\Uninstall\{AppId}\ (could be either HKLM or HKCU, so best check both) where {AppId} should be substituted with the actual value you used. Look for the UninstallString or QuietUninstallString values and use the Exec function to run it from your InitializeSetup() event function.