Using XP-SP2, IE7, Outlook 2003 and the HTML format, there is a significant typing delay creating new e-mail messages, both using stationary and not. Using text or rich text, there is no lag. Also, IE7 in general seemed slower.

I have eliminated anti-spyware and ant-virus as a cause. Office Detect and Repair did not fix anything.

Resetting IE7 fixes the problem.
Control Panel>Internet Options>Advanced>Restore>Reset.

Caution: this resets the IE7 add-ins also, which must be investigated and re-enabled as necessary.
Open IE7>Tools>Manage Add-ons>Enable or Disable add-ons.

I have traced the issue down to SpyBot Immunization and Spyware Blaster Enabling Protection for Restricted Sites. Either one causes the problem. This is repeatable. The problem started around 1/20/07.

In addition, I have found that Sun Microsystems BHO (ssv.dll) slows IE7.

Disable SSV Helper class - (It's from Sun Microsystems) It's a BHO - ssv.dll

Close browser and then re-open.