There’s a new version of Orca available (4.5.6001) that is suitable for Vista. You get it by downloading and installing the Windows Installer 4.5 SDK, available here (6.79 MB).
After the SDK is installed it will have delivered the installer for Orca into the program files folder (“%ProgramFiles%\Windows Installer 4.5\SDKTOOLS\orca.msi”). At this point you install it by double-clicking.
I recommend you archive the orca.msi file somewhere for the next time you need it.