I am guessing that if you are using a 4th generation PC and Windows 8.1, you are using an F200 or R200 camera? As the SR300 asks for a 6th generation processor and Windows 10.
If you have an R200 then you should use the '2016 R2' SDK instead of Essentials (also known as 2016 R3), as R2 was the final version of the official Intel RealSense SDK that supports the R200 (support continues in the open-source Librealsense SDK though). The latest 2016 R3 SDK is for the F200 and SR300 cameras.
I forget to mention that I'm using the F200 camera
This will sound like a silly question, but are you sure it was the 3D Scan install file that you ran? On the image that you kindly supplied, the title of the window with the 'no components to install' message is 'Essentials'. So I would expect to get that message if you had already installed the Essentials module.
It is not silly at all
I accidentally added different screen shot from other attempts to reproduce this error
(on different computer - the 3D Scan was installed and then I uninstalled the Essentials and try to reinstall it and get this message. I then uninstall the 3D Scan and could continue with a clean installation (first Essential and then 3D Scan)
this is the error that I could not get over
I assume that this model is partially installed ,maybe there are some leftovers on the registry?
I sometimes get this message when installing a DCM. I found that the previous DCM I had installed was not removed from the Programs and Features list, even though I had done an uninstall. I believe that once I had persuaded the DCM to finally remove from the Programs and Features list by telling it to uninstall again, the next attempt to re-install the DCM was successful. So there could be something in your theory about leftover files.
Thanks for the help ,unfortunately I can't uninstall it since its no longer appears on the Programs and Features list
If I have a problem like this, I often open the Deviice Manager, look for the camera under Imaging Devices and see what drivers are listed there. If all is well, it should list an RGB driver, a Depth driver and a Virtual driver. Sometimes the Virtual driver is missing, and this can cause problems with the camera's functioning.
Fixing a missing Virtual Driver in Windows 10 is usually easy - you can uninstall all the listed RealSense drivers (e.g RGB and Depth) by right-clicking on the driver in the Device Manager and selecting 'Uninstall', and then unplug the camera and plug it back in. Windows 10 then re-installs them automatically by downloading them from the internet instead of using the DCM to install them.
As you are using Windows 8.1 though, things may be more complicated. To restore a missing Virtual Driver, you would probably have to uninstall the drivers from Device Manager and then run the DCM installation program.