I was testing Symantec Endpoint Protection for a short while. After uninstalling endpoint protection I began receiving an error every time that I opened outlook. The error said something to the effect of “Unable to load Add-on please uninstall”.

In Outlook 2003 you should be able to simply remove the add-on within the add-on manager. In Outlook 2007 though it requires a different method. I had to delete a file called Extend.dat (location: C:\Documents and Settings\%username%\Local Settings\Application Data\Microsoft\Outlook) which is the file that stores the cached add-ons. After running Outlook again this file was recreated but this time Outlook did not give me an add-on error.  This seems to apply to other add-ons as well. While searching the web I saw people report that this also works for similar errors after uninstalling AVG antivirus.