OneCare should not be doing anything to a browser session that requires password entry vs. one that does not. Memory is being watched and files are being scanned as they are opened or saved to disk. What browser are you using and are you using any add-ins for that browser? If you are using add-ins, disable them to see if the problem goes away. I'd also recommend clearing the temporary Internet Files and possibly deleting all cookies.
Disable OneCare virus and spyware protection under Change Settings in OneCare to see if it goes away after the above don't resolve it.
Contact support for help if you believe OneCare is the culprit.
How to reach support (FAQ) - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
-steve