Given a long history of IE vulnerabilities we all know that every day in- day out some attacks are carried out using IE. Basically IE6 which you can find most of the users using even today in organizations by default. So why can't organizations replace IE with FF or Opera etc? All are free as well relatively safer than IE.
Here is a nice article I was going through. It analyzes the cause and and efforts given to replace IE. Surprisingly, after replacing IE with FF and Opera the calls to IT help desk for removing and cleanup of malware,spyware have gone down drastically!
More information read this: