Hello,
I have had the same issue today (Avast has blocked SIHclient.exe and I have accepted to move the file in quarantine).
Some additionnal information: yesterday, I have accepted a major update of Windows 10 (Fall creators update, version 1709) and after the update, Avast asked also me to restart my computer due to an update (I think an Avast update was ecessary for this new Windows 10 major version).
I my list of windows updates installed, I have seen other updated installed today; KB4057247, KB4054022, KB4055237 and KB4055994).
In the event viewer, I see also another updates installed a few minutes before the Avast alert;
- Installation démarrée : Windows a commencé l'installation de la mise à jour suivante (installation started of a windows update): 2017-12 Cumulative Update for Windows 10 Version 1709 for x64-based Systems (KB4054517)
- Installation réussie : Windows a installé la mise à jour suivante (windows installed the following update): 2017-12 Security Update for Adobe Flash Player for Windows 10 Version 1709 for x64-based Systems (KB4053577)
- Installation démarrée (installation started of a windows update): Windows a commencé l'installation de la mise à jour suivante : Update for Windows 10 Version 1709 for x64-based Systems (KB4058043)
So, for me, this alert is due to Windows Fall Creators 10 update, Avast update or the post-Windows updates following Windows Fall Creators 10 update...
For now, I have kept the file in Avast qurantine. Avast is uptodate (program and antiviral definitions), not sure if it is safe to restore these file, if this file is really needed and I haven't tried to repair Avast. A third Avast user (and maybe other users) has also add the same issue on december, 21st:
https://forum.avast.com/index.php?topic=212067.0 Regards,
Christophe