It depends if you want a Per-User install or a Per-Machine install.
The main difference is where the registry keys are saved, under HKLU or under HKLM:

The other main difference is Per-User is set by default to install in the Users folder and Per-Machine is set by default to Program Files.
As @Aron mentioned certain prerequisites may require admin rights to install. If you know the client has the correct .Net version installed its possible. Otherwise you might need to resort to running the VSTO Add-In without .Net. Note: Microsoft Info path is written entirely in .Net so if users have the full office install, .Net 2.0 (maybe more recent depending on the version) should already be installed.
The easiest way to avoid the UAC prompt is to run the MSI not the setup.exe. Because the setup.exe is responsible for the downloadeding/installing components (such as dotnet). Running the MSI wont ask for admin.
Also note the MSI command line usage to see all the switch's (eg /quiet):
Start > Run > CMD > Enter
c:\Apps\App1\Release\app.MSI /?