It's meant to do that!
The backspace key is a standard keyboard shortcut, to view the previous page, in most browsers (including Internet Explorer and Firefox). The only time that it shouldn't operate is when the cursor is active within a text field (e.g. when you're composing a post for AB).
If backspace is taking you back to the previous page, even when the cursor is within a text field, you probably need to re-install Internet Explorer. However, it would be far easier (and much better) to switch to a decent web browser, such as Firefox. (As previously stated, the backspace key is still a keyboard shortcut to the previous page in Firefox, but it definitely 'switches off' this function when the cursor is in a text field):
http://www.mozilla.com/en-US/
Chris