I have windows XP in my system but recently I am moving to Windows 7 and Internet Explorer 8 from windows XP/IE8, I locate that each few weeks my saved usernames on forums and other sites (for example banking) are purged as well as I have to re-enter them. I am not confident what the culprit is. I have the similar AV - kaspersky - on all my PCs and this merely happens on Windows 7.
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