Hiya Chris,
Just to add to the thread, I used XP Pro for many years until this year and used the HTML Editor the entire time without any issues (well few issues mostly of which my computer was just too old to handle things lol). I didn't have to run in any type of compatibility mode at all.
Have you tried reinstalling it? Also, if you haven't done a reinstallation try:
uninstall the program
Run Ccleaner which you can get at
http://piriform.com (free program to help clean temp files and remove registry entries from uninstalled files and such. Very trustworthy, many of us use it.).
Reboot your machine
Turn off Firewall and Scanner software
Reinstall the program
Turn firewall and scanner back on and see if it now works.
If you already tried the above then... never mind