Disclaimer: I am not very familiar with Windows (I run Linux here), but I saw the frustration of users like Bill, so I thought I would try to put together some notes for those who want to run the RX888 SDR with the HDSDR application on Windows, based on my understanding and several comments on different groups.

If any of these notes are incorrect, please feel free to comment below, and I'll change them accordingly.
Also if you are running Windows, and want to attach screenshots to help new users with the installation of all the components, you're very welcome to add them to this thread for future reference.

Anyhow, based on on my understanding, these are the steps to get the RX888 SDR to work with HDSDR on Windows (I tried to document them as best as I can - my personal approach here is not to go to the next step until you are 100% convinced that all the previous steps are OK).

Edit: Hayati has built an ExtIO dll that integrates both the Windows driver and the SDR firmware (see his post here: https://groups.io/g/NextGenSDRs/message/104) - using his built simplifies the detailed steps below significantly: you just need to complete steps 1 (Cypress USB driver) and 2 (HDSDR software install); everything else after that is included in Hayati's integrated zip file (https://codingspirit.de/ExtIO_RX888_v04_static_integrated-fw.zip).

1. The first step is the USB driver - for this one you will need the Cypress USB driver for FX3 for Windows. These drivers and the instructions can be found on Cypress website in KBA94413 here: https://community.cypress.com/docs/DOC-12366. Another very good source of information for this step is Simon's excellent guide here: https://www.sdr-radio.com/rx-666#Driver. Simon's guide has screenshots, and I again I recommend not to move forward until this part looks OK.
2. Download and install the latest version of HDSDR from here: http://hdsdr.de/. Alberto I2PHD who wrote this great software (another fellow italian!) has very detailed instructions on the site. After this step you should have a folder called 'C:\Program Files (x86)\HDSDR' with 7 files there (including one called 'HDSDR.exe', about 7.8MB in size)
3. Next we need to add the two files that allow the RX888 to run (the firmware), and HDSDR to be able to 'talk' to the RX888 (the ExtIO dll) - both files need to be copied to the HDSDR folder (i.e. 'C:\Program Files (x86)\HDSDR'):
   • the RX888 firmware is a very small (about 110kB) file called 'rx888.img' (the '.img' extension is typical for these files)
   • the current version of the ExtIO dll for the RX888 is called 'ExtIO_RX888.dll' and it should be about 1.6MB in size
   For convenience I posted a zip file with these two files here: https://groups.io/g/NextGenSDRs/attachment/98/0/RX888.zip (the source of the RX888 firmware is Justin Peng's post; the source of the statically compiled ExtIO dll is Hayati Aygün's post)
4. Since the ExtIO dll uses the FFT for down-converting and decimation, we'll also need the dll library for the FFT. The FFT library used is called 'FFTW' (http://fftw.org/), and the precompiled Windows dll can be downloaded from here: http://fftw.org/install/windows.html. The file you want to download is called something like fftw-3.3.5-dll32.zip. After downloading it, unzip it and you just need to copy the file 'libfftw3f-3.dll' (should be 2,391,615 bytes in size) to the HDSDR folder mentioned above.
5. ** IMPORTANT: this step is not needed if you are using the statically compiled ExtIO dll referenced in this post ** If you use a different ExtIO dll, that was built expecting the Visual C runtime to be already on your computer, you may get a cryptic message that says "The specified module could not be found" (see Bill's message below).
   To fix this error, you need to add the 32-bit Visual C runtime ('VCRUNTIME140.dll'); install the required "Microsoft Visual C++ Redistributable for Visual Studio", which can be found here: https://support.microsoft.com/en-us/help/2977003/the-latest-supported-visual-c-downloads - there you'll see a link to download a file called 'vc_redist.x86.exe' (make sure you choose the x86 version since the dll is 32-bit); download it, run it, and it should take care of the required Visual C runtime.

At this point I think we have everything we need; we just have to connect the RX888 to a USB 3 port on the computer, and HDSDR should be able to see it and use it.

Hope these quick notes help (and of course feel free to post your comments to make them better).
Franco