Have you enabled the option to read everything in Plain Text? If not, do you
scan incoming messages with an antivirus scanner? The latter can prevent
Outlook from rendering HTML because some of the information in the message
gets stripped out by the scanner.
If you scan incoming mail, uninstall your AV program, reboot, and reinstall
the AV program without the mail scanning featiure. You will be just as safe.
Note that this will not fix messages you have received already, but it should fix
new incoming messages.
Brian Tillman [MVP-Outlook]