Such objects run in the same memory context as the browser and can perform any action on the available windows and modules. For ex., a browser helper object can install hooks to monitor messages and actions, etc.
Some BHOs are helpful, like the Adobe Acrobat or Google Toolbar Browser Helper Objects, but there could be malicious ones among them that will harm your computer, especially those planted by viruses or spyware. BhoScanner allows you to discover browser helper objects of your computer including parasites and trojans. BhoScanner gives you a quick look at the BHOs installed on Local or Remote PC, tells you by coloured icons ( green icon - safe, yellow icon - unknown, red icon - harmful ) whether specific BHO is known to be safe or harmful.