You can use an external HTML editor and then add the resulting html document to your stationary folder...
Compose your email in your editor of choice and save to the Stationery folder:
C:\Documents and Settings\%username%\Application Data\Microsoft\Stationery (Win XP)
You can then use this as a template and send mail from the Outlook menu:
'Actions > New Mail Message Using > More Stationery...' and selecting your file.
HTML email can be very problematic due to the limited rendering capabilities of various email clients, particularly Outlook 2007 and GMail, so it's good to keep the email structure simple (use tables for layout - like back in the bad old days), use redundant formatting (inline as well as in the
<head> etc.) and to test send to different types of email account before sending 'live'.
A lot of useful info about formatting and other aspects of HTML email can be found at the MailChimp site.