I did a boot full scan with avast free, four file in cache folder of chrome and opera cannot be scanned because of cyclic redundancy check. Could these be virus and using this way to prevent scan? Even I use windows 10 scandisk command with fix disc then avast boot full scan again, same result.
Empty your browser cache.
I did this already then full boot scan, same result.
If you want/need a check…
Attach your basic diagnostic logs. (MBAM, FRST and aswMBR)
Instructions: https://forum.avast.com/index.php?topic=53253
I will do this after I get back to home, thank you.
OK, you’re welcome.
also post a screenshot of what it is you see
screenshot of avast scan result or other?
avast scan result
In the scan resut page, the file location is too long and I can just place the mouse cursor over there to display one by one, anyway to display all file location full path?
Follow instructions posted by Asyn and attach requested logs
The screenshot of Avast boot full system scan is attached. 3 files having the same warning “cyclic redundancy check”.
I do the 3 suggested scanning now and then I will attach the log.
It is located in windows old folder 
http://www.howtogeek.com/223821/what-is-the-windows.old-folder-and-how-do-you-delete-it/
Yes ha ha I can see it now ha ha I will just delete the whole old windows stuff and do full scan again.
after removed old windows files then full scanned, reslt shows no virus anymore, thank you!
By the way, what could be a reason why Avast showed error for those 3 files virus in old windows!?
No idea, and nobody will/can track it down.