Same problem. I hit alt+ctrl+del to see if it is an application still running. If not I log off and log back on. One of these days we'll get a PC/System that works.
Me and my wife and the same issues (2 computers). It was random and often. We upgraded to IE8 about a month ago and no problems since.
I think the issue comes often because of tabbed browsing. I know that if a tab <<crashes>> in IE 8, the tab will just close but the rest of the program will still be running perfectly.