Mozilla 1.5, for web, email, news, and calendar
I stopped using IE at 5.0. I got sick and tired of downloading the lastest weekly security patch at windows update. If you don't turn off all javascript, cookies, and active x garbage you leave yourself wide open to the latest and greatest IE/Outlook security/trojan/virus/you get the idea.
I also support the moz project because it is multiplatform, supports more w3c standards and promotes web desiners writing intelligent standard code rather than microsoft crufty html.