I have downlaoded the drivers explicitly made by Ati for the iMac (not desktop nor mobile); I am afraid I will never get anything by the next bootcamp update since my iMac is considered a deprecated systemThat's a valid point. Your iMac may not be capable of running Windows 10 if Apple does not update Boot Camp for your machine.
In that case, I would revert to running Windows 7 which is a rock solid version of Windows, or stick with 8.1. I'm not especially impressed with Windows 10 but that's just my personal opinion. In that case, I would revert to running Windows 7 which is a rock solid version of WindowsWindows 7 mainstream support has already ended, moreover newer versions of Visual Studio support it just partialy; it is not a valid choice. [SOLUTION] At the end I have succeded making Windows 10 video drivers working: I have first installed Windows 7, then bootcamp and finally I have updated to Windows 10. During Windows 10 installation the resolution has been reverted to the minimum one (something like 1024x768, I'm not sure; anyway installing Windows 10 directly the generic Microsoft video driver was able to bring to 1920x1080) and then Windows succesfully downloaded and installed the needed driver from Windows Updates.
Windows Vista. Select PowerColor Video card ATI Radeon HD 4670 Series PCI-E driver for download. Select only OS AMD / ATI High Definition Graphics Driver. View details. Version, Date: 8.65,. Supported OS: Windows XP x32. Driver Hp Scanjet 5590 Windows 10.
Since the driver is the same I think that probably there was some kind of conflict with other drivers, probably the Win 10 display one. The only big difference (but it should be completely uncorrelated) is that before I was using a disk partitioned using GPT and now an hybrid MBR (btw Win 8 and 10 can not be installed directly on an hybrid MBR disk because they are able to detect GPT and so they want a GUID partition). Anyway thanks for the attention.