"Library not registered" or "Interface not supported" error

An application we have that automates MS Office Apps Powerpoint (in this instance) is installed in hundreds of clients offices worldwide.  In one particular client we have just encountered a unique situation where the app crashes on running.  After a few debug versions we found that the line that causes the crash is:

var
    PowerPoint: PowerPoint2000_TLB.PowerPointApplication;
...
    PowerPoint := CoPowerPointApplication.Create; // Exception "Library not registered"

PowerPoint itself on this machine seems to work fine, its just trying to start an instance programatically that fails.

One or two machines on this clients site work fine but several do not.  We cannot find what is different about these non-working machines.  We have uninstalled Office and reinstalled from CD (then applying latest hotfixes), unplugged the machine from the network and logged in as administrator locally, inspected the installation for anti-virus, COM add-ins, other plug-ins and nothing seems different.

We have tried things like regsvr32 ol32.dll and powerpnt.exe /regserver to no effect - we are stumped.

Be grateful for any other things to try...

Richard Teller