I use IE because I've been using it since Windows 95 days. I hate it when people are standing over my shoulder watching me do something and they look all surprised that I'm not using Firefox, then try to preach how good it is at me. Firefox may be more secure, but I feel I'm a savvy enough computer user to not go to dodgy sites, or even if I do, I have enough experience to deal with to consequences.
I have both installed, but IE just feels "right" for me. Not IE7 though, the tabbed browsing interface uses so much RAM its unbelievable.