I'f you're looking for something powerful, and more importantly, free, take a look at Vim. It's built into most versions of Unix and linux, but you can download it for Windows as well.
http://www.vim.org/
It's a very powerful text editor with sytax highlighting. But it can also to a diff from explorer. You just have to highlight both files, right click, and select Diff with Vim.
Hope it helps.