How to get a dialogue to fire up a second dialogue

Question

We have a dialogue box that gets fired up using showDialogue(), this then fires up a second dialogue box when the user changes a combo box, again using showDialogue(). When this second dialogue is closed it throws a:

An unhandled exception of type 'System.Runtime.InteropServices.SEHException' occurred in system.windows.forms.dll

Additional information: External component has thrown an exception.

Any ideas what causes this?

ta muchly.
b.