Many users of Lotus Notes, myself included, at times get the error message “An Error Occurred While Opening a Window“. To resolve this, all processes that belongs to Lotus Notes have to be terminated.
From IBM, there’s a file available called “Killnotes.exe”, which does just that. The file can be obtained from Lotus.com. Simply unzip and run the file, to terminate all the processes. You should then be able to start Notes without a hitch.