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Avast Free Antivirus boot-time scan

By , Last modified: 2015-02-28.

How to schedule and perform Avast Free Antivirus boot-time scan in Windows XP, Vista, 7, 8 and 8.1

Boot-time scan is an effective way of detecting and removing malware, because the scan takes place before Windows starts up and therefore rootkits and other system modifiers have little to no chance of hiding themselves.

However, there are two important caveats:

  • inexperienced users can easily make Windows unbootable if they allow avast! boot-time scan to delete or move system files,
  • boot-time scanning takes a long time - up to several hours.

For these reasons, you should always use RKill first to stop malware processes and services and then run a full anti-virus scan and a full anti-malware scan. This is a much safer and often a quicker way to remove rootkits and other nasty hard-to-detect malware.

Avast Rescue Disk (bootable USB drive or CD/DVD) can do offline scanning and cleaning if some rootkits run even before avast! boot-time scan starts.

If avast! Free Antivirus 2014 displays the following dialog after scanning or cleaning, it is strongly recommended to leave the message open, run RKill and then perform a full Malwarebytes scan (links in previous paragraph). Only then it is safe to answer Yes to "Do you want to schedule the boot-time scan and restart the computer now?".
Avast Free Antivirus 2015, Do you want to schedule the boot-time scan and restart the computer now? Click 'Yes' only after running RKill and performing a full anti-malware scan.

Scheduling or unscheduling Avast Free Antivirus 2015 boot-time scan

Open Avast 2015 main window by clicking its yellow circular icon in Taskbar Notification area (aka System Tray). It is also recommended to save all open documents and close all running programs and apps.
Avast Free Antivirus system tray icon, click the yellow ball to open main window.

Open Scan tab from the left side of the window and click Scan for viruses.
Avast Free Antivirus 2015, Scan tab. Select 'Scan for viruses'.

Open the list of scans near Start button and click Boot-time scan.
Avast Free Antivirus 2015, Scan tab, Scan for viruses. Select 'Boot-time scan' from dropdown list.

"Boot-time scan" is now listed and set as "not scheduled". Click Settings.
Avast Free Antivirus 2015, Scan tab, Scan for viruses. To configure boot-time scan, click 'Settings'.

Boot-time scan parameters window opens. By default, System drive and Auto-start programs (all users) are set in Areas to scan section. Replace System drive with All harddisks if you have multiple hard drives or partitions.
Tick the Scan for potentially unwanted programs (PUPs) and Unpack archive files check boxes.
From the When a threat is found, apply the following action combo box, select Fix automatically. This will let Avast choose the best possible action if infection is detected, and it will not delete important system files.
You should never choose Delete or Repair here - first one might irrecoverably erase important system files; second one will leave most of malware intact (there is nothing to repair!), stating "Error 42060 (The file was not repaired)".
Click OK to close the scan parameters window.
Avast Free Antivirus 2015, Boot-time Scan Parameters. Select 'System drive' in the 'Areas to scan' section. Set 'When a threat is found, apply to following action' to 'Fix automatically'. Then click OK.

Now, back in the Scan tab, click Start button to run the scan right away.
Avast Free Antivirus 2015, Scan tab, Scan for viruses. Click 'Start' to run a boot-time scan.

Avast Free Antivirus 2015 will confirm your decision, click Yes to restart your computer now.
Avast Free Antivirus 2015, Do you want to restart the computer now? Click Yes.

To discard the boot-time scan, click Delete Scan instead.
Avast Free Antivirus 2015, Scan tab, Scan for viruses. Click 'Delete Scan' to cancel a boot-time scan.

Boot-time scanning with Avast Free Antivirus 2015

You can stop the boot-time scan at any time by pressing Esc key on your keyboard. Windows will start up as usual after this. But I do not recommend doing this unless you really need to - your computer might still be infected and you should reschedule the scan to clean the PC.

This is how the boot-time scan looks like. All infected files that are not required by Windows will be moved to Chest.
In case Avast detects an infected system file that is critical for running Windows or is located in Windows folder or its subfolder, it stops and waits for an action to perform. In such case, always press number 3 on keyboard to skip moving the file.
Do not use the Yes or Yes all options for system files - your computer might become unstable or unbootable! You should use Rkill and Malwarebytes Anti-Malware for repairing such infections after the scan has finished and Windows starts. ComboFix is also available for Windows XP, Vista, 7 and 8 (but not 8.1) users as a last resort.
Avast Free Antivirus 2015, Boot-time scan, File is in Windows folder, are you sure. Press '3' to skip moving files that are in Windows folder.

As said before, the scanning might take up to several hours. After the long process is complete, an overview of the results is displayed for a few seconds and Windows will load.
Avast Free Antivirus 2015, Boot-time scanning complete, continuing with boot.

If you want to see the full log of boot-time scan just performed, read about Avast Free Antivirus logs here.

Troubleshooting if Windows does not work properly after Avast 2015 boot-time scan

In case you experience problems with starting or running Windows after a boot-time scan, you can follow these steps to get your PC running properly again:

  • If Desktop is empty and no Taskbar appears - this means that Windows/File Explorer process did not start properly. Use keyboard shortcut Ctrl+Shift+Esc to launch Task Manager, open File menu and click New Task (Run). Type explorer and click OK. Desktop icons and Taskbar should appear now. If not, continue to next steps.
  • Start Windows in Safe Mode, then open Command Prompt and run sfc /scannow. This will restore essential system files.
  • If launching Windows in Safe Mode is not possible, Windows XP users can run Non-destructive reinstall from Windows installation CD.
    Windows Vista, 7 and 8/8.1 users can boot from installation DVD and click Repair your computer (see the Repair your computer in Windows Vista or 7 and Repair your computer in Windows 8 and 8.1 articles for more detailed instructions). Windows 7 users can also use System Repair Disc for booting; Windows 8/8.1 users can use Recovery Drive for the same task.
    Try Startup Repair/Automatic Repair first. If this is of no help, use System Restore - but keep in mind that this might also restore the malware; you must run a full virus scan again to remove it.
  • If repair options are not helping, restore Windows from the latest backup (you do have one?). Windows 7, 8 and 8.1 users can see the Restore a System Image article for instructions; AOMEI Backupper and EaseUS Todo Backup tutorials for restoring disk image backups are also available.
    No backups? Use Puppy Linux for rescuing your personal files and reinstall Windows.



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