It appears to be a Windows error message.
When you get an "illegal operation" error message, click the "Details" button, and post back with the exact details word for word, up to but not including the memory registers.

Signature
Glen Ventura, MS MVP W95/98 Systems
http://dts-l.org/goodpost.htm
> An obnoxious pop-up keeps appearing at the most
> inopportune times: ...illegal operation, close, see your
> program vendor.... Makes no sense and I have never had
> this situation before. What can be done to get rid of
> this "OBNOXIOUS" POP-UP? Thanks.