Configure your email client (program) so it's not open to attack by malicious email. Simply viewing a message will trigger the attack if your program is not set up correctly. Attachments are a big threat too, but that's another matter.
Attacks by email rely on malicious code imbedded in HTML messages. HTML is the same format used for webpages, and email clients work like browsers. The attack can easily infiltrate your computer, install spyware, set up a Trojan horse, or turn your computer into a zombie.
Even with tight security settings, you'll still need to be cautious. Learn more at Step 6: Learn to Handle the Dangers Online.
If your email client is not Thunderbird or Outlook Express, you'll need to set it up using them as an example. Or look online. ;-)Instructions for Thunderbird
This is easy: By default, Thunderbird will not allow a virus or worm to execute automatically. There are a couple of things to do to tidy up. [Go to the security settings instructions for Thunderbird.]Consider upgrading Outlook Express. Microsoft now offers Windows Live Mail, which is more secure than Outlook Express. [alternate link -- look for "Betas"] These instructions for Outlook Express will work with Live Mail too. (Find options for Live Mail at the right end of the toolbar, next to the "Help" button (blue question mark).)[more]
Instructions for Outlook Express