It has been going on for years, and it happened to me again. I got this error after upgrading to 2012 when I used the Burst command.
ACET Autoload not found, blah, blah, blah.
Well, Burst is an Express Tools command, so number one make sure Express Tools is loaded. Seeing it in the ribbon is a good indication. If it is not loaded, go to Programs and Features in the Control Panel, and select the current version of AutoCAD, then select the Uninstall/Change button.
From there change the install to include Express Tools. See the image below.
If all of this doesn't solve your problem, chances are, you are just missing a path in Options. Under the Files tab make sure a path like the one highlighted below is present.