Installer service problem

I have one particular XP Pro box, out of several, that has a problem with it’s Installer Service. Here’s the message –

The Windows Installer service could not be accessed. Contact you support personnel to verify that the Windows Installer serviceis properly registered.

I am guessing that refers to registered in the Registry. I noticed on my machine that the Installer service is set to manual startup and is not shown as running, yet installs work fine. On the one box installing one particular app fails, even after starting the service.

What do I do to fix this?