Installing SharePoint Server 2010 with a trial license is something that doesn’t happen often. Since we did install SharePoint trial edition we ran into an issue which I wasn’t aware of:
Trial editions and licensed editions of SharePoint, Project Server, and Office Web Apps products may not be installed on the same server.
The screenshot below shows the error message you get, directly after running the setup.exe.
For those who are wondering how to work around this, you can’t. The only solution is getting a license key and upgrade the Trial license to a Standard or Enterprise license. Afterwards you can install Office Web Apps.
Tip: Being aware of this I strongly recommend using the Standard or Enterprise license during installation instead of afterwards.