Spaces vs. Tabs are generally annoying when editing files, but start programing in Python and this turns into a complete disaster. Simple solution, always insert Spaces instead of a Tab and you are good to go (my personal preference):

set tabstop=4set shiftwidth=4set expandtab

If you want to be really fancy:

:retab

Will convert all of the existing Tabs into Spaces.

Check out Converting tabs to spaces on the VIM Wiki.