Windowless ActiveX controls are not supported

I recently started to see this intriguing exception message from a WinForm application that we write. After much googling, I came across a number of other blog entries suggesting a wide range of issues, none of which helped much. This is code that has worked for years, but had suddenly ceased to do so!

In the end it turns out that the underlying ActiveX dll had become unregistered on the system, so it was failing the COM initialisation process. An interestingly obscure error message for this condition, but a quick regsvr32 call later and it all worked again!