BHO's- Browser Helper Objects.

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.

Some companies go out of their way to hide the presence of the spyware BHOs that they install. They go so far as to find ways around the most popular detection tools by changing their product regularly just enough to avoid detection until the next version of the detection software comes out.
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