I encountered this problem when creating a new signature in Outlook 2003 on Vista and then when I press the Advanced Edit button, the error message is "Could not find an editor for H (.) files"
I tried some suggestions from the forums then merged them together and finally solved the problem.
*Note: Make it sure that you are in ADMINISTRATOR account before doing this step or if you are in local user accounts, make that user as an administrator first. Then after doing the steps, make it as local user account again.
Here are the steps:
1. Control Panel> User Accounts
2. Click 'Turn User Account Control on or Off
3. 'Uncheck the box with a tick in it marked 'Use User Account Control to help protect your computer'
4. You computer will then need to restart.
5. From IE, choose Tools--Internet Options--and click on the Programs tab.
6. Change HTML editor to Wordpad/Notepad. Click apply and OK.
7. Now, change the HTML editor again--this time, back to Microsoft Word. Click apply and OK.
8. Open the advanced editor in Outlook 2003, add your signature with special fonts, images etc. Save and close the advanced editor program you used, clicking OK or Yes to all questions to get out of the Options Menu and back to your Outlook home page.
9. Test to make sure it works.
10. Then shut Outlook 2003 down and go back to Control Panel> User Accounts
11. Click 'Turn User Account Control on or Off' again
12. This time check the box next to 'Use User Account Control to help protect your computer'
13. Your computer will restart again.
14. Open Outlook and test again. It should work fine.