I finally installed a new ROM on my HTC Diamond, its a considerable old phone, but since my older one broke I have to use it. I hate to waste time on this but it was done in a few hours.
The straight forwed steps are:
Upgrading to HardSPL
Download OliNex Unsigned Hard-SPL (Developers Edition) from here;
Connect the phone with Microsoft Active Sync
Run the installer
Install the ROM
Download the ROM from here
That is it, my phone is working awesome now.