Firefox makes an excellent replacement for Internet Explorer. It provides greater compatibility with Web standards and also has a much better security record.
Click here to download Firefox.
Thunderbird is an email program made by the same people, and it is a good replacement for anyone who is using Outlook Express.
Click here to download Thunderbird.
Firefox and Thunderbird also run on Mac OS X, so you can use them to replace Safari and/or Mac Mail as well!