Strange that there is a crashlog.tar.gz in the windows\system folder, .tar and .gz are forms of zipped file, it certainly is not a windows zip style that I am aware off.

Could it possibly be that it originates from some third party system tool or even an online scanner that compresses virus definations in a .tar.gz format in the hope not to confuse anti-virus programs? The name however would point to the first guess.

I would tend to rename the file first, rather than delete (especially if avast is not reporting it), you can always delete it if after some time there is no ill effect.

The Lsass.exe is a system file - Local Security Authority Service
. I don’t think that it is connected with the file.

http://www.liutilities.com/products/wintaskspro/processlibrary/lsass/

Hopefully you are coming to the end of your journey.