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Suddenly the scan completes with the notice that it cannot scan some files. This has happened occasionally and it would be a few or several files, and they generally fit the description in Avast Help, e.g. archive files. However, suddenly the scan has started to report that there are literally hundreds of files that can’t be scanned. They are mostly located in a temp files folder. I can’t tell what kind of files they are. But given the drastic change in scan results, combined with somewhat degraded performance on the computer, I’m concerned about malware that is escaping Avast’s scrutiny. Is there anything I can do about this?
Welcome to the forum. 
You could start by cleaning up the temp folder and getting rid of those files by using CCleaner.
If they continue to return, you may have an infection and should follow the instructions outlined at:
[b]https://forum.avast.com/index.php?topic=53253.0[/b]
How do I handle files that avast! can’t scan? http://blog.avast.com/2014/02/28/how-do-i-handle-files-that-avast-cant-scan/
TFC - Temp File Cleaner by OldTimer http://www.geekstogo.com/forum/files/file/187-tfc-temp-file-cleaner-by-oldtimer/
But given the drastic change in scan results, combined with somewhat degraded performance on the computer, I'm concerned about malware that is escaping Avast's scrutiny. Is there anything I can do about this?If so, follow instructions here https://forum.avast.com/index.php?topic=53253.0
I’d like to say my long delay in replying is due to having solved the problem, but it’s actually due to many distractions of recent life coupled with the fact that the computer continues to function more or less OK most of the time. The “less” is an occasional extreme slow-down, nearly a freeze, in performance, which seems to be caused by a 100% utilization of the disk. The Task Manager informs me that the disk indicator measures the “percentage of time the disk is processing read or write requests.” Frankly I don’t know what exactly that means; I don’t recall a similar measurement in the Task Mgr of Win.XP. Usually I shut down a few programs and the problem subsides after 10-15 min. I think the onset of this problem coincided with the inability of Avast to scan the aforementioned temp files. They are all in one folder:
C:\Windows\Temp\92c2323f-604e-402b-834f-45e97f718ea1\Windows\WinSxS
There are over 2,400 sub-folders there, each of them containing about 1 to 3 files, with extensions such as .loc, .inf, and .sys; totaling about 11,800 files. I have used cCleaner on these files but it has no effect, even though I specified this file location in its custom settings. Of course I tried to delete them directly, but this too is impossible. I’m met with the error message: “folder access denied: you’ll need to provide administrator permission to delete this folder.” I do have administrator status and am in fact the only user of the computer (shared with my wife but only one account).
At the end of every Avast full scan it is always these 11,800 files that are unscanned. The Avast error message is: “Error: the file cannot be accessed by the system (1920)”
If there is malware causing this, apparently it’s not especially malicious if this is all it does (and it’s been 4 months). But it’s still a nuisance that should not have to be tolerated, and makes the continued use of Avast scarier. Furthermore, Avast - which I’m paying for, and will have to pay again in a week as our license expires - is not doing its job. The suggestion given me here previously is to follow the instructions on the web page:
https://forum.avast.com/index.php?topic=53253.0
That page instructs me to download and run one or more, perhaps several, anti-malware programs to run in addition to Avast. This raises the question of the efficacy of Avast in its purpose to protect its users from malware. It seems unreasonable for designers/providers of AV programs to issue a product that is inadequate in itself and requires supplemental software to compensate for its defects.
I have downloaded Malwarebytes’ Anti-Malware and will install and run it in hopes that this will solve the problem. This is partly because we used MBAM before in conjunction with another AV program with some beneficial effect. I would rather not need to go beyond the use of MBAM, unless it is perhaps to supplement cCleaner with Temp File Cleaner - but here again we are cluttering our computer with a surfeit of software.
We’re inclined to continue using Avast rather than have to embark on a potentially maddening experiment with yet another AV program. On the whole it seems like Avast has performed better than our previous AV programs over the years (Norton, McCaffey, and AVG). But this situation is both alarming and vexing.
Thank you for any further assistance or enlightenment that may be offered.
TFC - Temp File Cleaner by OldTimer http://www.geekstogo.com/forum/files/file/187-tfc-temp-file-cleaner-by-oldtimer/
When the work is done, you can delete it
We're inclined to continue using Avast rather than have to embark on a potentially maddening experiment with yet another AV program. On the whole it seems like Avast has performed better than our previous AV programs over the years (Norton, McCaffey, and AVG). But this situation is both alarming and vexing.Did you uninstall these? Did you run vendors removal tools to clear all leftover files?
General: Uninstalling a third-party antivirus software http://www.avast.com/en-us/faq.php?article=AVKB11#artTitle
All those other AV programs were on an older computer. Since getting this new laptop we’ve used nothing but Avast.
See if this answers your question:
http://blog.avast.com/2014/02/
Yes, I read that item in the beginning, but it seems irrelevant. There have always been an occasional few files that were not scanned, but they were understandable in terms of the reasons given in this article, and evidently innocuous. However, as I’ve explained, the current and persistent batch of nearly 12,000 files appeared suddenly and coincided with the intermittent degradation I described previously. The explanation in the article doesn’t seem to adequately address my situation. Also, this enormous group of inaccessible files is in a temp folder, yet is apparently permanent. That’s one of the reasons why I suspect mischief is afoot.
Follow instructions at the bottom of my first reply…attach the diagnostic logs from Farbar Recovery Scan Tool