There are sometimes circumstances in which you need a more or less specialized version of the browser. Sometimes you work around this by developing a completely custom module built on top of the WebBrowser control, complete with buttons, labels, and whatever else the user interface requires. In this case, you're free to add to that browser any new, nonstandard feature you want. But what you actually have is just a new, nonstandard browser. The WebBrowser control is just the parsing engine of the browser. This means there still remains a number of UI-related tasks for you to do: adding an address bar, toolbar, history, status bar, channels, and favorites, just to name a few.
So, to create a custom browser you have to write two types of code: the code that transforms the WebBrowser control into a full-fledged browser like Microsoft® Internet Explorer, and the code that implements the new features you want it to support. Wouldn't it be nice if there was a straightforward way to customize Internet Explorer instead? Browser Helper Objects (BHO) do just that.

Internet Explorer is just like any other Win32-based program with its own memory space to preserve. With Browser Helper Objects you can write components-specifically, in-process Component Object Model (COM) components-that Internet Explorer will load each time it starts up. Such objects run in the same memory context as the browser and can perform any action on the available windows and modules. For example, a BHO could detect the browser's typical events, such as GoBack, GoForward, and DocumentComplete; access the browser's menu and toolbar and make changes; create windows to display additional information on the currently viewed page; and install hooks to monitor messages and actions.

If you're running the Microsoft Windows 98, Windows 2000, Windows 95, or Windows NT® version 4.0 operating system with the Active Desktop™ Shell Update (shell version 4.71), BHOs are supported also by Windows Explorer.

A "Browser Helper Object" is a DLL that allows developers to customize and control Internet Explorer. When IE 4.x and higher starts, it reads the registry to locate installed BHO's and then creates them. Created BHO's then have access to all the events and properties of that browsing session. The APIs for building BHO's are very cool -- they give developers almost complete control over Internet Explorer.

Applications which install BHOs are becoming more and more popular because BHOs allow application developers to control Internet Explorer. For example Alexa uses a BHO to monitor page navigation and show related page links. GetRight and Go!Zilla use BHO's to monitor and control file downloading. Flyswat, Quiver, Blink, iHarvest, etc use BHOs to extend and control Internet Explorer. BHO technology has allowed the development of some very powerful (and cool) applications.

BHOs don't require a user interface per se, though many install Internet Explorer toolbars. Therefore, its possible that there are BHOs installed on your system that you don't know about. What this means is that while there are some really good uses for these things, they may not necessarily need your permission to install and they can be used for malicious purposes like gathering info on your surfing habits.

A lot of spyware and BHO's are written quickly and poorly. This can cause anything from incompatibility issues to corrupting important system functions making them not only a threat to your security but to your systems stability. The programmers of spyware applications obviously do not care about you or your system other than as a source of marketing information so they do not error check most of their products.

To check for BHOs run HijackThis.