was I hacked? or was this from a plugin? see code please

Hello,
I noticed that my site wasn’t loading right, and when I try to open it in IE, I get an error and my Avast anti-virus freaks out, and tells me that there’s an infected script. After messing with the database, pulling some hair out, etc., I went to the theme editor in the dashboard, and noticed that all of the theme files have tons of jibberish at the beginning:
[Code moderated. Please do not post hack code blocks in the forums. Please use the pastebin]

All of the pages should start with:
<?php get_header(); ?>
Right? And not have all of that junk before it? Do you think I was hacked?
I also recently was messing with some popular download manager plugins, but those wouldn’t have inserted something like that, would they?

Sincerely,
Heather