My suggestion for 'installing' is to use ExcelDnaPack to put everything you need into a single .xll file. Then your users just copy it anywhere they want, and add it to Excel through the add-ins dialog (press Alt+t, i). It feels like cheating but works
There are other options, but this would certainly be the best start.
To use ExcelDnaPack you just set
<Reference Path="..." Pack="true" />
<ExternalLibrary Path="..." Pack="true" />
and then run
This gives you a MyAddIn-packed.xll which needs no other files, and no admin privileges to run, install or to add as an add-in in Excel.
* You can add a self-registering bit in the AutoOpen. Then users only have to open your .xll by double-clicking, and it will stay added in for future sessions.
* Some developers try to make more sophisticated setups for their add-ins. I think it's mad and can't really help, but you don't have to do much more then copy the files and run some script to register.
* Making a setup for "All Users" is nearly impossible.
* Some developers make their own update manager tool (as another add-in) that can dynamically look for and load new versions of the add-in.