First try this... when configuring any HTML nugget, there is an option labeled "Perform Markup Transformations" - set that to NO, and save your configuration.
kleinerChemiker's suggestion was to disable wysiwyg mode if you should be using it. You will find this setting under "personal settings". The option is labeled "Use WYSIWYG editor". You can disable the wysiwyg editor completely by setting this option to "No".
If you do not want to disable the wysiwyg editor (if you are, in fact, using it), you can disable automatic entry of HTML by the editor by using the button that looks like "<>" (without the quotes).