When an ATL ActiveX control that subclasses an edit control is placed in Internet Explorer, pressing the BACKSPACE key while the focus is on the Edit control causes Internet Explorer to go back to the previous web page. If there were no web pages earlier than with the control, the BACKSPACE key functions as usual.
When a user clicks on a subclass element, the control is not activated. This means that any accelerator keys are handled by an Internet Explorer frame. Controls are never given changes to handle the BACKSPACE accelerator key.
refer: http://support.microsoft.com/kb/190044