Windows 8.1 update : help needed.

[#1]

Since installing update KB3000850 this morning I've been having serious problems. I'm using a Lenovo G780 laptop running Win 8.1, fully updated. Since installing the update, startup takes about 5 minutes instead of one, the laptop will not sleep and will only shut down if I do it by holding down the start switch for about half a minute.
When I try to access the appropriate Items in the control panel to adjust the power switching etc. or go to the windows update pages to try to delete KB3000850 the control panel freezes.
I tried System Restore but it stayed on" System restore is initialising" for nearly half an hour until I shut down.
I'm VERY relucant to reinstall as I assume I would have to go through updating from 8 to 8.1 all over again and would rather just bang my head against a brick wall.
Can anyone offer any better suggestions please?

[#2]

Very mixed comments about it..
Quite a few people are saying it interacts badly with Avast!, so maybe if you are running that it is worth changing to something different for a bit (if performance will let you).
~~~
I just installed the update (despite the risks) and it seems to have gone smoothly for me.  I had been an Avast! user, but changed to Windows Defender after some Avast! javascript performance issues.
Somewhat after the horse has bolted for you I am afraid, but I use EaseUS Backup Free for regular backup.  (I used to use Acronis for the same purpose, but that started behaving very badly at one point so I gave up on it.)
They both mean you can recover a complete installation from the backup point.  Much more space consuming than System Restore; but work when the system is somewhat broken (like yours) or even if it gets completely broken.

[#3]

This update is showing as 'optional' so I won't be installing it at present.

[#4]

Thanks for that - maybe I should try going back to Win defender for a bit.
I have been using Aomei backupper for a while but I'm not sure I know how to restore from it  (correction. I'm damn sure I don't know that but maybe I might know someone who does - at a very moderate price.)

[#5]

Re the above - does anyone know what special steps, if any, need to be taken when uninstalling Avast?

[#6]

When I uninstalled Avast! I didn't need to do anything in particular, all very clean.
I may well reinstall it some time, it does have better rating than most for detection.
Good luck with getting machine running properly again, problems like that are such a wasteful hassle.

[#7]

Thanks sjptd.  When I've had a go at it I'll report back with the result.

[#8]

Holding down the shift key, click or tap on Restart.  The computer should restart with Advanced Startup Options.
Choose 'Troubleshoot'.
Choose 'Advanced Options', then 'System Restore'.
System Restore is far more likely to work if you start it this way.

[#9]

If you can then boot into safe mode, it should let you get through the desktop process of uninstalling avast, failing that boot from the install dvd and select the recovery environment instead of installing windows again from there you should be able to uninstall the latest update and then start windows normally in order to get rid of avast

[#10]

SUCCESS ! This morning I decided to take what I hoped would be the easiest route out, and Revo uninstaller got rid of Avast for me. After rebooting everything now seems to be back to normal. Phew.
As I am fairly conservative with my web browsing and other computer use I shall probably stick with Windows Defender for the forseeable future.
For now, my thanks to everyone concerned for their help and advice.

[#11]

One more question - is it safe to install  KB3000850?

[#12]

Not with Windows 8xx. 
I've just finished reading through the Avast forum and loads of people seem to have the same problem. Try this link.
https://forum.avast.com/index.php?topic=160717.0

[#13]

Thanks to advice here I installed the small security update and left KB3000850 well alone.
Have been quite happy with Avast but have now to decide whether it goes.

[#14]

@Roberto0151, thanks for the link. It seems the problem is caused by Avast/ KB3000850 combination. Wonder if MS are aware of it

[#15]

Is this just Microsoft, getting sneaky? and doing something to make AVAST do this, so that people eventually only rely on Microsoft Defender ?