I've been trying to install an add-in for Word 2003 (with all necessary updates I believe) for Serenity's Editor software which has its own add-in installer. The add-in appears to install properly but when I open Word, the error message comes up: Exception from HRESULT: 0x80040200 and there is no sign in Word of the extra software. I run XP Home SP2. Can anyone help me get this sorted?

Exception errors are from the program receiving a value it cant handle.

So this is a bug in the programing your installing contact their support and send them the error Exception from HRESULT: 0x80040200 they can easily check the code and tell you what is going on.

When SetSecurity custom action is invoked - one of the parameters that is passed in is a target directory where add-in is installed. The code in SetSecurity.cs checks whether this target directory is not empty and, if it is, it will throw an exception with the text you are quoting above. You probably need to double check the parameters you are passing to the custom action - there is quite a few so it is easy to mistype something.