Try again
Try again Keygrip, and you can get it to work (assuming it is outlook) (and by the way a system restore won't do aything to help.
in your normal mail view, right click on one of the column headings (such as From, and select customise current view.
Then click on Group by, (should be 2nd entry down) and tick automatically group, and select expand all by default.
OK your way out of that and you will have extacly what you want.
If the display is not quite what you expect, check tools/calendar options/first day of week. For instance, if this is set to Sunday, and today is Tuesday, then the grouping would go Today, Yesterday, Sunday, Last week, etc.
If First day of week was set to Monday, then the display would go Today, Yesterday, Last week, etc. and the dusplay will change every day - comprendez?