Sure.
Setup.exe is the setup bootstrapper - it starts the whole process, and loads
whatever is needed.
The instamsi files are for windows installer. If you don't have windows
installer on your system (if, for example, you are using an older operating
system) then these setup.exe will install windows installer first, and then
run the .msi file to install the software you intend to install in the first
place.
There are two versions: instamsia and instamsiw. The a stands for ASCII, and
the w stands for wide characters. You can think of a as meaning Windows 9x
and w as meaning Windows NT/2000/XP. The reason is that Windows 9x handles
all characters as single byte ASCII values, which is fine if you speak
English but not so great if you live in a country with a much larger
alphabet than will fit into a single byte, such as the far east countries.
NT/2000/XP handles all characters as Unicode characters (and if you provide
ASCII values, it will just convert them for you before using them), which
are two bytes long (hence the designation wide) and able to handle many many
more characters.
Once setup has determined that you have windows installer (either already
installed or installed by one of the instamsi files, depending on your OS)
then it runs the msi file(s) for your program, which invokes the windows
installer to provide a much more robust installation experience than letting
every software developer out there write their own installation package, and
more than likely doing a poor job of writing the uninstallation package.
