After update of my windows 10 I got problem Desktop is unavailable

Yes, you can, and maybe it is best what you can do. I made also clean installation after 3 days of trying to save old system. Firt of all, I made backup of old user folder via Ubuntu boot disk. Then I used media cration USB>F12 boot from USB> New installation>Format system disk>Installing windows>Installing other software and their updates again and then setting. I lost 36 hours of clean time of my life due to Microsoft and probably Avast.

OK now I really have a problem, my laptop won't boot up at all. I was stuck at 55% of the installation using the media creation USB etc for over 5 hours, and thought it was never going to work so I stopped the install and tried again, this time it won't boot and just gets stuck in a loop: boot screen>black screen>brief flash of a blue screen saying 'installing updates 0%'>black screen>boot screen etc etc. I can use F2 to get into setup and F12 to get into multiboot but that's it.

Can I just reformat the HDD and start fresh?

Hope someone can help, I'm entering day 4 of my Update nightmare.

I am an IT guy and basically do this sort of thing everyday. This is about the 4th or 5th update Microsoft has released since 10 came out that has massively broken thousands of windows computers all over the world. I can always tell when Microsoft just released an update because my computer repair company gets flooded with calls of messed up computers and PC's that wont boot.  

Yes in this case, there are ways to get windows back in working condition( the option to download the latest major release on to a USB stick and do an install while keeping files is the easiest for most im sure). The fact that THE END USER is the one who has to take action and spend hours upon hours filled with stress and anxiety experimenting with workarounds because Microsoft has taken away the option to NOT receive updates is insane. Yes you can defer them if you have Pro and Yes you can configure your system services if you know how to do that without screwing something up. But the fact that they force you to take their broken updates over and over again costing us time, energy, and stress does not speak well for the company concern for its customers. 

This is all coming from someone who ultimately benefits financially from Microsoft's continuous update blunders. If I didn't have basic moral standards, I'd be thanking them for providing our business with more repair work than we can handle. Please Microsoft, get it together and stop breaking your own product. I am and have been an avid outspoken supporter of your platform for years. I defend you for every major windows release and wish for your growth in the market.  I'm afraid if this continues It'll make it impossible for myself and others to have your back and promote your brand. I do hope these messages get to you and you'll do the right thing by making changes to how you roll out these updates.