I amn on windows 7, action center notes that both windows defender and eset nod 32 4.2 are running, and suggests this may slow the machine considerably. So which, if either, should I disable?
Neither. They are different applications and run perfectly side by side without affecting your system.
The note about multiple applications is generic and does not cover your situation. If you were running Eset and MSE or Eset and Norton together yes you would have a problem.
Windows Defender will not interfere with an antivirus application.