I've never tried to make an installer. Wix (http://wix.sourceforge.net/) would probably be the tool I'd use.
There are two tricky issues:
* Detecting 32-bit vs 64-bit Excel and installing accordingly.
* Installing the .xll add-in as an Excel Add-in means you need to write an entry into the OPENXXX key, where XXX is incremented for each add-in. So you need to run some script to check the registry and loop through values until you find the first number
If you make something it would be a nice contribution to Excel-DNA - please let us know. If you can wait until later in December, I can help also and we can figure it out together.