We downloaded and installed the IBM Informix OLE DB Connect drivers v2.81 from IBM. Points to watch during installaton:

Check that the path file points to a valid Informix directory - my path pointed to a previous Informix installation that had not been removed properly.

Also check that the services file in WINNT/System32 has an entry for the Informix service - the installation does not appear to write this to the services file. Make sure that you include a carriage return if you add the reference to the services file at the end of the file!