Taken from here:
Browser Helper Objects - Browser plug-ins which are designed to enhance the browser's functionality. Entries can be found in the registry at:
HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Explorer\Browser Helper Objects
Toolbars - Additional toolbars that appear in a browser, often below the address bar. Entries can be found in the registry at:
HKEY_LOCAL_MACHINE\Software\Microsoft\Internet Explorer\Toolbar
URLSearchHooks - Used when an address without a protocol such as http:// has been entered in the browser's address bar. Entries can be found in the registry at:
HKEY_LOCAL_MACHINE\Software\Microsoft\Internet Explorer\URLSearchHooks
Explorer Bars - Internet Explorer sidebars located adjacent to the browser pane. Entries can be found in the registry at:
HKEY_LOCAL_MACHINE\Software\Microsoft\Internet Explorer\Explorer Bars
HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\Explorer Bars
Additionally, custom items for context menu:
HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\MenuExt\<Menu Text>
HKEY_LOCAL_MACHINE\Software\Microsoft\Internet Explorer\MenuExt\<Menu Text>
Custom items for Tools menu, custom toolbar buttons:
HKEY_LOCAL_MACHINE\Software\Microsoft\Internet Explorer\Extensions\{GUID}