To all on this thread (and this is from personal experience): do not muck around with the default themes. Leave them alone, do not replace them. If your active theme fails, WP will have nothing to fall back on. Then you will be in a panic because your blog is borked.
When you upload a theme, make sure the files are all in its containing folder. (ie: wp-contents/themes/name-of-theme) No theme files should be alone in the wp-contents or themes folders.
Make sure all necessary files are present:
index
header
footer
sidebar
style.css
Most will also include more such as post, single, archive, comments, search, etc.
Open style.css in a text editor to make sure it has the standard header info:
/*
THEME NAME: xxx
THEME URI: http://xxx
DESCRIPTION: xxx
VERSION: 1.0
AUTHOR: xxx
AUTHOR URI: xxx
*/