You can export the note from the Windows client as a .enex (Evernote Export format) - then open that file in your favourite editor (it's just XML) and fiddle with the HTML; then import it back into Evernote. Be careful of the structure though!
Alternatively, the web version of Evernote uses a customized version of TinyMCE editor; these support having events injected, and you can inject the mceCodeEditor event to popup an html editor for the current note. Obviously it relies quite heavily on the implementation of the Evernote web page, and isn't supported in anyway by the Evernote team!
If you want a bookmarklet that does exactly that (i.e. I've done it for you!), see here
http://seb.so/html-source-editor-for-evernote-web-a-bookmarklet/ for more details