Samboll,
I recently upgraded from 2.5.1 to 2.9.2. The blog works fine but the admin page is messed up. I put a screenshot of it here.
I re-uploaded the /wp-admin and /wp-includes folders again and double-checked the wp-content folder to ensure I didn’t have extra files. I then went through and ensured all old files were updated except for user-specific files.
Any further ideas?