Notepad++ is an advanced text editor for Windows, with HTML/XML tag highlights, and an enormously good search and replace feature. Great for correcting mass spelling errors, formatting issues, highlighting a bunch of tags, etc... In some ways I find it better than MS office.
It's used for programming, and NOT WYSIWYG!
Very handy for trimming down unnecessary data from ebooks.
It's probably the best tool to create or clean up Epub files (or other html based ebooks).
Download it from the :
Notepad++ Home Page