It is flagged as a PUP by MBAM - Potentially Unwanted Program - if it was on your system and you had no knowledge of it and or didn't install it. Then it could be considered unwanted (PUP).
The lack of a culprit in the Task Manager is normally as a result of the system hanging for some reason, hard to pin down when the system doesn't give you much help in the Task Manager.
If this issue happened after the avast update, then it might be worth doing a clean reinstall, Download the latest off-line installation file, see
https://forum.avast.com/index.php?topic=189836.0 for direct download links.
- Download the avast! Uninstall Utility, avastclear.exe
http://files.avast.com/iavs9x/avastclear.exe and save it to your HDD.
- 1. Now uninstall avast! - using add remove programs/Programs & Features, if you can't do that start from the next step, reboot.
- 2. run the avast! Uninstall Utility, it will request it be run from safe mode (and will handle the boot into safe mode), once complete reboot into normal mode.
- 3. install the latest version, reboot.
Note:1 if you can't do this in this order because your is using 100% CPU then try booting into safe mode and uninstall from there and then run the uninstall utility.
Note:2 save the copy of avastclear.exe so it can be reused as some users are reporting that after running it, it is self-deleting.
As far as the previous AV, not knowing exactly what it was makes it difficult to point at an uninstall utility by the maker to ensure all remnants are gone.