Internet Explorer 8 Frame/Session Merging
By default IE8 merges sessions across all open browser windows. This behaviour prevents you from, for example, having two IE windows open simultaneously that are each logged into different Windows Live accounts.
If you want to start IE reverted back to the IE7 and earlier behaviour of not merging sessions across browser windows, use the –noframemerging option:
Start –> Run: iexplore –noframemerging
