Thank you.
This is how I operate:
1 - open windows explorer
2 - go to the iso file to be burned and double click.
3 - imgburn opens with the source selected.
4 - I close windows explorer to be sure
5 - click on write.
Then I have the msg and I push continue and the burn seems to be made correctly as I can browse and execute the burned files.
As far as I see, no other program which could use the burner is running.