Error: Cannot modify header information

Can someone please point me in the right direction?

I installed a vanilla WP 2.0.1 on a vanilla database on a vanilla server (hosted by dreamhost). When I went to activate the plugins, I got the following error:

Warning: Cannot modify header information – headers already sent by (output started at /home/.sagan/daria/unsympathetic.net/wp-content/plugins/adminmenu.php:311) in /home/.sagan/daria/unsympathetic.net/wp-admin/theme-editor.php on line 59

I get the same error when I switch themes, it’s in the place where the “upload” function should be on posts and pages, and when the page reloads after editing the theme, or adding a catagory.

Whatever I’ve asked it to do gets done, but it’s really annoying to have it there, and then have to go back and around to see the changes.

I’ve installed 2.0.1 on a localhost, on my other domain, and a work domain, and I’ve never seen this error before. I didn’t edit any files before uploading.

And direction given in resolving this error would be greatly appreciated.

(And if this error has been discussed before, I’m sorry. I tried to search the “error” pages, but every page but the first one only brought up the tag cloud for me.)