Did you look for it in the Windows Control Panel > Add & Remove Programs??
It might have gotten installed by a program that you downloaded as an "extra" and might not be called "Bing". Be suspicious of anything that you don't recall installing on purpose, it might masquerade as a "Search" or "assistant" program.

You could also look at your installed extensions - { Ctrl + Shft + A } > Extensions.
If you find any that you don't recall installing yourself, click on those one at a time & hit More and you'll be able to see the date that extension was installed or last updated.

Remove or un-install it when you locate it.

You can open the about:config page via the location bar and do a search for bing via the Filter at the top of the about:config page.
You can reset all bing related prefs that appear bold (user set) via the right-click context menu to their default values.

• http://kb.mozillazine.org/about:config