It's amazing how often a seemingly innocent action by the user and an update for a program can affect negatively another. In this case, it's MS07-027: Cumulative Security Update for Internet Explorer, released in mid-May. After installing that update, Outlook 2003 responds very slowly when typing HTML messages.

The problem is two-fold. First, users either added a large number of sites to IE's restricted zone or installed a "security" application (of the spybot type) that added them. The cumulative security update then changed the way the Internet Explorer 7 rendering engine interacts with the Restricted Sites zone and this change causes Outlook to pause after each keystroke while the IE7 rendering engine checked the sites in the zone.

The user has several options to fix this issue:

1. Remove the security update.
2. Switch to plain text formatted messages. Using RTF message format will fix the problem but should only be used when all of your recipients use Outlook.
3. Remove all of the unnecessary sites from the Restricted zone.
4. Use Word as the email editor.

Of these options, removing the security update should be the last resort. Editing out the chaff in the Restricted zone would be my first choice since too many users load it with sites they are unlikely to ever come across and which may no longer be "live". Reset the IE security defaults to clear the list quickly.

MS07-027: Cumulative Security Update for Internet Explorer
http://support.microsoft.com/?kbid=931768