[Setup]
; no ArchitecturesInstallIn64BitMode directive is used here, Inno Setup is so in 32-bit mode

[Code]
const
  CrystalReportsKey = 'SOFTWARE\SAP BusinessObjects\Crystal Reports for .NET Framework   4.0\Crystal Reports';

...
begin
  // if we are on 64-bit system
  if IsWin64 then
  begin
    // if the key is found in 32-bit registry view, it means that a 32-bit CR is installed on the
    // system (if the CR does it so; that's what I don't know); HKLM32 == HKLM in 32-bit mode Inno
    // Setup, so you can write HKLM here (but HKLM32 better explains what is done here)
    if RegKeyExists(HKLM32, CrystalReportsKey) then
      MsgBox('32-bit Crystal Reports found.', mbInformation, MB_OK);
    else
    // if the key is found in 64-bit registry view, it means that a 64-bit CR is installed on the
    // system (that's the case on your system)
    if RegKeyExists(HKLM64, CrystalReportsKey) then
      MsgBox('64-bit Crystal Reports found.', mbInformation, MB_OK);
  end
  else
    // we are not on 64-bit system, so there is only one, 32-bit view, which is mapped as HKLM in
    // 32-bit mode Inno Setup
    if RegKeyExists(HKLM, CrystalReportsKey) then
      MsgBox('32-bit Crystal Reports found.', mbInformation, MB_OK);
end;