Outlook Express, Outlook and Internet explorer are currently dangerous. There are currently 24 unpatched Internet Explorer/OE security holes, some enabling a full compromise of the target PC. The most common exploits install keystroke loggers and spam forwarding software onto your PC. PCs can be compromised by "promiscuous" web browsing with Internet Explorer or by viewing rouge email in Microsoft Outlook Express or Outlook.
Here is some advice from CERT on the matter, probably the most important reference is to
http://www.cert.org/tech_tips/malicious_code_FAQ.html
Another alternative is to install the Firefox browser from
http://texturizer.net/firefox/index.html which provides better functionality as well as better security than Internet Explorer.
A secure alternative to Outlook Express is Thunderbird
http://www.mozilla.org/products/thunderbird/