[Setup]
AppName=My Program
AppVersion=1.5
DefaultDirName={pf}\My Program

[Code]
const
  ChromeRegKey = 'Software\Microsoft\Windows\CurrentVersion\App Paths\chrome.exe';
  FirefoxRegKey = 'Software\Mozilla\Mozilla Firefox';

// based on http://stackoverflow.com/a/22538486/960757
function IsChromeInstalled: Boolean;
begin
  // check if there's the Chrome app registration entry under the 32-bit registry
  // view of HKCU or HKLM key
  Result := RegKeyExists(HKCU32, ChromeRegKey) or RegKeyExists(HKLM32, ChromeRegKey);
end;

// based on http://stackoverflow.com/a/7135283/960757
function IsFirefoxInstalled: Boolean;
begin
  // check if there's the Firefox reg key under the 32-bit registry view of HKLM
  Result := RegKeyExists(HKLM32, FirefoxRegKey);
end;

function InitializeSetup: Boolean;
begin
  // we won't continue if either of these is installed
  Result := not (IsChromeInstalled or IsFirefoxInstalled);
  // if we won't continue, show up a message box
  if not Result then
    MsgBox('You have installed Chrome and/or Firefox. Uninstall them and run this setup again.', mbError, MB_OK);
end;