newbie,

This is usually caused by a corrupted file association.

Technical’s recommendation is what I would also recommend.

If not successful, then try the other steps Technical stated.

I would also perform a DiskScan and Defrag just in case the error is created in the bootup sector. Sounds like I am putting the cart before the horse…You say “but it is not booting up but shutting down!”
Not true. A corruption in the boot area of the disk can have an effect on the shutdown order of the open processes.

Simply put, the “shutdown” command for Avast gets lost.

It is not that difficult to isolate and repair.

Techie