It is always advisable to have an extra browser installed (beside an anti-virus software) incase any virus is caught on online during surfing the internet. An extra browser would be very useful since some virus will freeze up the Internet Explorer browser.
Two different browsers are always installed on my laptop. It saved me lots of troubles when the virus locked the IE browser. The backed up browser allows me to access to the internet and search for solutions.
The latest statistic (Feb 2007) shows that IE is still the lead browser used by surfer. Hackers will rather target to the highly used software (example: Microsoft). In this case, IE have bigger chance of being hacked.
I've changed to FireFox from IE the past couple months. Compare the two, FireFox browser open time is faster, it has a restore website function if the browser is accidentally closed, smaller installation file when download, etc.
The best is to keep both browsers, since some website still required IE to view.