Auto Install VBA Code for Excel

I wrote some VBA code in Excel. I then saved it as an add-in. When I re-opened Excel, and went to the Add-in manager I found that I had to check the box beside it to make it load.

Since I will be writing and distributing Apps written in C# that collaborate with the VBA code in Excel, it would be very useful if that code could be installed and enabled for loading without any manual intervention after the Add-in is placed in its folder. How might this be done?

I assume there is folder for the Add-In where it will be loaded for each user on the machine and each new Excel workbook and each existing workbook. What folder is the proper one for say Excel 2003?

BTW: The Apps and the VBA code will be installed on one machine and nothing will require a network. Assume there are no security restrictions for running the code. The code is event driven and not accessed by the user.

I have seen this done by a text-to-speech converter that installed some time ago. After installing the App, I looked at the Add-Ins with the add-In manager and there was one with its checkbox already checked. Furthermore, for every user and every workbook, that VBA was loaded and ready to use. How was that done.