Basically what I'm trying to achieve is the following:
If the user run the setup with the /SILENT
or /VERYSILENT
parameters, the setup will immediately present the EULA. If the user rejects, the install is canceled. If the user accepts, the rest of the install will happen in silent or verysilent mode.
Edit: both solutions presented by RobeN and TLama worked perfectly. The only problem is when the EULA is too big to fit a Message Box (that would be the most common situation). Anyway that's a good solution to at least display some warning or information before the install begins.