I believe this started when I once had Secunia Inspector mistakenly set to automatically install Opera Updates - the first time it did Opera was reset as my default browser. Since then it resets to default every time I update. I tried using the Custom installation but didn't see the selection to disable that. Have to try again next time and look more carefully.
BTW, I had to uninstall IE9; it was working OK but too many programs stopped working well. E.g., Ultra Recall Pro, IQ, and a few others. Back to IE8 till it gets sorted.