Well, since I'm a tech freak at my school, I use all the best stuff. Basically everything my IT department doesn't support, such as SP2. 

 Boy, am I glad I upgraded when the Sasser virus came around. 
The above was 

. Now onto the real stuff.
IE has 
WAY too many security holes. I only use it for Windows Update. Since Mozilla is like Netscape on steroids, I don't get nearly half as much crap as my classmates. I've even converted a few over to Mozilla. Also, Mozilla has limitless features, such as tabbed browsing etc.
SO  MICROSOFT!!
 MICROSOFT!!