A BHO is a COM.DLL that allows developers to customize and control internet explorer.  When it starts it reads the registry to locate installed bho's and then creates them.
The API's for building BHO's are very cool but the potential for abuse is high.  BHO's give give developers almost complete control over internet explorer.  However, because BHO's don't necessarely have a user interface, it's possible that there are BHO's running on your system that you don't know about.
Tools to control BHO's :
BHO CAPTOR puts the use in control.  It's a free application that lets you control the internet explorer Bowser Helper Objects that are installed on your system.
BHO COP is similar to BHO CAPTOR but shows more detailed information and has the ability to activate/deactivate BHO's at will.
BHO DEMON is another one even better


     A comprehensive look at BHO'S, what they are and do, etc.


    Tony Klein keeps a list of BHO's