Uninstall OpMAnager Trial version

HI there,

I installed OpManger Trial version. It worked fine. I wanted to uninstall the software but that was not possible. I decided to remove manually the entire folder C:\ManageEngine\OpManager.

Now, I want to reinstall the software (free version) but I have an error message telling me that I need to uninstall the previous version. A screen of the the dialog box is here:
<a href=" https://imgbb.com/"><img src=" https://image.ibb.co/maWruv/Op_Manager_Install.jpg" alt="Op_Manager_Install" border="0"></a><br /><a target='_blank' href=' https://fr.imgbb.com/'>free image hosting</a><br />

My question is: is there any tool that uninstall properly OpManager, including windows services? Many thanks in advance. Regards.