Here is where to get hold of version 5 of Orca – the one most suitable for Windows 7 able to validate .msi files for Windows Installer 5. It is in the Windows SDK for Windows 7 and .NET Framework 4. Unfortunately there’s no mini-download for the msi sdk like with version 4.5.
You need to install the debugging tools from the Windows SDK, then go in to C:\Program Files\Windows SDK\7.1\Bin and orca.msi will be in there.
[Update 2012-09-08: there’s a slightly newer version available here]