How long have you been using IE9? It sounds to me that this is the culprit. Have you set up what it cleans out and what it leaves? I keep IE9 only because without it you can't get tghe Microsoft updates. My main Browser is Firefox and as of yesterday 6. I too use Yahoo to do my browsing. But the cleaner has always been CCleaner, for many years and I don't have any trouble with lost login details.
Fujitsu Lifebook P727 laptop review