From Yahoo! News:
The Department of Homeland Security's U.S. Computer Emergency Readiness Team touched off a storm this week when it recommended for security reasons using browsers other than Microsoft Corp.'s Internet Explorer.
Personally, I use Mozilla Firefox. It's in version 0.9.1, so theoretically it is in beta, but I have no problems with it. It is as easy (if not easier) to use than Internet Explorer, it (of course) lacks all the security holes present in IE, and it includes many more features, such as Tab Browsing, which lets you open multiple web pages in tabs at the top of the screen rather than opening a pile of new windows for each page. It's very nice. It's also much faster than IE, and it is more standards-compliant--so it almost never shows pages wrong.
You can also get Mozilla Thunderbird, which is an email client to complement Firefox. (Mozilla still has a full-blown suite, and Firefox and Thunderbird are standalone, faster, better versions of the browser and mail components of the full suite. They really are much better, they're not just part of the full suite in a separate program.) While it won't replace Outlook, it could probably replace Outlook Express, and it is much more secure than Outlook (which isn't very hard to be) so you won't spread email worms.