You’ll find the answer at:
https://forum.avast.com/index.php?topic=176180.msg1250122#msg1250122
I’m having the same issue. It started with the free version of Avast!. Now that I’ve purchased the Avast! Internet Security, it will give me the Critical Error each time. If I remove Avast!, then it works fine.
I was an Win 8 → Win 8.1 → Win 10 upgrade process.
Did you follow the directions already outline here:
https://forum.avast.com/index.php?topic=176180.msg1250122#msg1250122
This is a Win10 error.
Create a task and run as admin to reboot in safe mode.
Then when its in safemode untick ‘boot to safe mode’… then reboot normally and the error should be gone.
Win10 seems to have a lot of little bugs like this.
You think its bad for you? I can’t even run Cortana here, as it is not enabled in my country, and yet Cortana still crashes many crucial parts of my O/S and/or desktop. When I am not using it. WTF and stuff.
I get the same error “start menu and Cortana cannot start” from time to time.
Some may blame 3rd party Antivirus for causing the trouble. I wouldn’t be so bold to claim what was NOT proven but, nonetheless, it gave me the clue.
Solution:
- Start your computer as usual
- Go have dinner, a jog, a shower… stay away from the computer for as long as you could afford. Allow whatever malicious tasks running in the background to complete. :-\
- Login, preferably, when the busy hard-disk LED stops blinking
The actual cure as found on the Microsoft help forum states to simply boot into Safe Mode with Networking
Once in Safe Mode, reboot into Regular Mode and the problem will be history. 
Booting into Safe Mode and then back into normal mode is NOT the end all fix to this problem! It does not always work.
Are you using the latest Windows 10 version and Avast too?
This was an issue in early releases, not anymore…
It’s still a problem with everything up to date. Everything on my computer is updated to the most recent versions…Windows 10 and the newest Avast Internet Security. I experienced the “Start/Cortana” critical error this morning. This is the third time this error has happened on this computer since the upgrade to Windows 10 a couple of months ago.
Looking at the Microsoft boards, I don’t think this is an Avast! issue, but it happens to Avast! users as well a lot of others. I probably jumped the gun upgrading the operating system. I broke my rule of waiting until the 1st service patch, but as annoying as it is, I can deal with it.
As others have said, the Safe Mode restart method does not always work. I’ve ended up just trying different things until I find something that actually does work for the current problem.
Like others, I am hoping a patch is found soon to put the problem “in the rear view mirror”.
You are right the other fix is to use the latest version of Avast and the latest version of Windows 10. (Windows 10 ver 1115 -Fall update)
It´s completely terrible, but this issue on W10 1511 (Fall update) still exists…
You appear to be the only one with the problem. ??? ??? ???
Hi malone.m,
thanks for reporting. Can you, please, post a screenshot of the issue (easily done with WIN+PrtScreen shortcut when a modal dialog/band is displayed). To us it appears that MS has fixed the crashing of the sihost.exe process in such a way, that it no longer displayes the mentioned critical error message, just restarts.
Lukas.
I’m also having this issue. This is the second time. I uninstalled Avast the first time and it was fine. Windows 10 is up to date. I even have Cortana and all of the location, privacy stuff disabled. I decided to take another shot and download Avast again a couple months later. And now I’m having the issue again. I can’t even uninstall Avast because it freezes on the first part regarding “aswNdisFit.” I’m not sure what to do. I have school stuff on here to do and am freaking out a bit. At this point, I just want Avast gone so I can use my computer again.
Hi startrekkin58,
I'm not sure what to do. I have school stuff on here to do and am freaking out a bit. At this point, I just want Avast gone so I can use my computer again.Freaking out for a good reason?: Maybe your school stuff is not backed up yet? That would be the first thing I suggest doing and then use this tool to completely remove avast and any other a/v programs previously installed prior: https://www.avast.com/uninstall-utility & https://singularlabs.com/uninstallers/security-software/ Win 10 version and threshold update number?
Surely not, according to MS feedback app.
Check PM please.
It appears that even doing a clean install of TH2 aka 1511 does not fix this problem
garyleung Dec 1, 2015 at 11:05 AM
I did a clean install this time with build 1511 hoping that would clear that error but today I got the same thing again. :\
The link you posted in your reply refers to versions prior to TH2 (1511). I personally haven’t come across this problem for quite some time.
The boot to Safe Mode with Networking and then a reboot into regular mode always worked in the original 10240 version of Windows 10.