Windows Booting very slow after 2004 update ,

Hi @Jaavenor - thanks for the feedback, I hope they are successful with your repair/fix at Geek Squad.

As regards the Lenovo Desktop PC and the 0xC1900101 - this can be several causes - check if you get a "sub-code" like 0x20017 after the main error code, for instance 0x20017 can mean it is a driver issue. Graphics drivers have caused trouble with several Feature Upgrades in the last, and I am sure that certain (older or out of date) Intel and NVIDIA drivers can cause update issues. A sub-code of 0x30017 might mean that a third-party Anti-Virus program is blcking some key part of the update process. I did see that there was a big list of Lenovo models that have issues with 2004 update, and I don't think they have all been resolved yet. I suspect that many models will still have driver conflicts. Have you checked the Device Manager list to see if any devices have yellow flags?

You can download the Microsoft update log analysis tool from the link below, install it and run it after an update failure, and check out its report. The sub-codes it finds might give you more insight, that you can pass to Lenovo support for advice.

https://docs.microsoft.com/en-us/windows/deployment/upgrade/setupdiag

There is a list of stuff here, many of which I suspect you already tried, but ignore the item 7 for "chkdsk" as they got the command syntax wrong anyway (no space after the command and the /f switch option, so you would get an error)!

https://support.microsoft.com/en-us/windows/get-help-with-windows-10-upgrade-and-installation-errors-ea144c24-513d-a60e-40df-31ff78b3158a

Thank you.  I delivered the "password amnesia" suggestions to the Geek Squad.  They accepted with thanks but of course I've heard nothing back from them.  Meantime I'm without my laptop going on a cumulative three weeks.  Appreciate your words.

Many thanks.  Apart from annoying "password amnesia" my Asus laptop is running fine, almost like new with a new SSD replacing the old HHD including a reinstall of all former apps.  I now have a problem with my Lenovo Desktop "Infocentre."  It fails to install 2004 generating error code Oxc1900101.  I've tried to do all of the Microsoft suggestion relative to this code; updated all drivers, disabled antivirus and disconnect all peripherals to no avail.  Every night the PC downlads and installs 2004 with a re-start request awaiting every morning.  This has been going on for months - I keep hoping that Microsoft might have some magic fix with each download and install.  I will try the refresh install and see if this allows update to 2004.  Thank you.

We have had loads of machines update with the 2004 update and nearly all are OK. Had one yesterday the really misbehaved itself.  (Dell Optiplex 3070, 8GB RAM, 256GB SSD.)

Once started the system would work ok for about a minute and then things got very slow. I Tried all of the above, nothing worked. I tried rolling back updates and features, this couldn't be done.

I uninstalled MS Office and Antivirus , OneDrive, etc etc and took everything off to bare Windows10, still the same: very slow:

Eventually, the machine gave up and wouldn't start properly and booted straight into the Trouble Shooting menu, I tried the usual, Startup repair etc, Wouldn't work. 

I had no option but to Reset Windows10. When I did this the machine was definitely alot faster, but I still wasn't happy with it, there was something not right, so I removed it from the domain, and deleted the existing user profile, downloaded and ran the 2004 update, rejoined the domain, reinstalled all programs and now it feels like a good solid installation again.

In this instance the 2004 caved the machine:

In my opinion from a general user viewpoint ,  never disable the SysMain service and the indexing service. 

With all of the computers we have updated with the 2004 update, to only have ONE go bad so far, this is not a bad record.

So, if you have this issue, the most time cost effective way to sort it is to do a REFRESH install, (Keeping personal files only). This way you will be confident that the job is a good one!

I did the same to my Asus laptop - reinstalled 2004 that later updated to 20H2.  I also installed a SSD.  All went well until Microsoft 365 Outlook, each time launching, decided to ask me for a password for each of my four email addresses, Onedrive and pin number.  This would continuously happen - even sometimes wile I was using Outlook.  Not even Geek Squad has figured it out.  I delivered the laptop to them for the third time - have not heard from them in a week.

That's brutal, my MS365 still working as it should be only asked my password once after the re-install. I guess I dodged that one. Good luck on your quest to repair this. :)