Today, I started Windows in my father's PC and saw with Task Manager there was the Avast installer working, installing itself with no permission.
He had Avast until months ago, but recently he changed his AV for another one, just to give it a try. Of course, he uninstalled Avast before installing the other AV. But now, Avast just appeared there and installed itself like nothing happened. His new AV alerted me that avastnm.exe installed things in Windows. There were 3 tasks, the main was located in C:\Users\XXXX\AppData\Local\Temp\ns2A4A722A\4902DA8E_stp, named avast_free_antivirus_setup_online__5_.exe. The other ones were located in C:\Users\XXXX\AppData\Local\Temp\_av_iup.tm~a06760 and C:\Users\XXXX\AppData\Local\Temp\_av_iup.tm~a06760\New_c0308e8.
My father didn't touch anything, nor I.
Now, Avast is there, in "pasive mode".