I pressed the “cnv>” button – why I can close Calc?
There is a bug in the way the “No Units Selected” warning is displayed when you click on the “cnv>” button with no units selected. If you don’t dismiss and instead click on another calculator function it is possible for this dialog to hang around in the background. Then every time you click the close button it pops up briefly and brings Calc to the foreground again. You can get rid of it by clicking on Today on the Start Menu which then allows you to get rid of the dialog.