It has always annoyed me that Emacs uses a mixture of tabs and spaces for indentation. People have told me this behavior can't be turned off, but -- lo and behold -- it can:

Emacs normally uses both tabs and spaces to indent lines. If you prefer, all indentation can be made from spaces only. To request this, set indent-tabs-mode to nil. This is a per-buffer variable, so altering the variable affects only the current buffer, but there is a default value which you can change as well. See Locals.