In Drupal, inserting "files/" directory under "sites/default/" breaks the theme/css

Question

I have a Drupal site that I upgraded from Drupal 5 to Drupal 6. The users' profile pictures no longer display because they were contained under sites/default/files, and there is now no "files" folder under sites/default.

I wanted to make sure the imagecache module still worked, but it didn't -- when I tried to upload a picture Durpal told me it "could not move file". So I made a "files" directory under "sites/default/" to fix it. However, not only did this not work, it caused the CSS of the theme to not render, making my site a mess of plain old html.

After experimenting around, I found that nothing bad happens when I put a folder named "files" under any other directory (including "sites/all/"), and nothing bad happens when you put a differently named folder under "sites/default", but if I put a folder named "files" under "sites/default/", it causes the CSS to not render.

So my question is, what is causing this and how can I fix it? How can I have the directory "sites/default/files/" without causing my theme to not display?