Boot-time scan is the most effective way of detecting and removing nastiest malware, because the scan takes place before Windows starts up and therefore rootkits and other system modifiers have little chance of hiding themselves.
Please remember that boot-time scan takes quite a long time - up to several hours.
Scheduling or unscheduling avast! Free Antivirus boot-time scan
Open avast! Free Antivirus main window by clicking its orange icon in Taskbar Notification area.
Open Antivirus tab and select Scan Now tab. Scroll down the list of available scans until Boot-time scan becomes visible.
First, click Schedule Now. The button text turns into Unschedule to let you know that the scan has been enabled. As usual, clicking the button again will disable the boot-time scan.
Then click the More details link right under the Unschedule button. Then click Settings.
Boot-time scan parameters window opens. Select All harddisks from Areas to scan. If you have multiple large hard drives, you could also select System drive here to cut down scanning time.
Tick the Unpack archive files check box if it is not enabled already.
From the When a threat is found, apply the following action combo box, select Move to Chest. This will move infected items to a special protected folder, but it will still ask you in case an infected system file is detected. You should never choose Delete or Repair here - first one might irrecoverably erase important system files; second one will leave most of malware in tact (there is nothing to repair!), stating "Error 42060 (The file was not repaired)".
Click OK to close the scan parameters window.
Now, back in the scans list, click Restart computer button.
First of all, 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 must reschedule the scan.
This is how the boot-time scan looks like. All 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, it stops and waits for an action to perform. In such case, always press number 6 on keyboard to try repairing all important system files. If the Repair all option fails, use option number 7 - Ignore.
Do not use the Delete or Move to Chest options for system files - Windows might not start after such actions! You should use ComboFix or Malwarebytes Anti-Malware for repairing such infections after the scan has finished and Windows runs.
As said before, the scanning might take up to several hours. After the long process is complete, an overview of the results is displayed and Windows will load.
If you want to see the log of boot-time scan just performed, read about avast! Free Antivirus logs here.
Troubleshooting if Windows does not work properly after avast! 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 Explorer 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 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 articles for more detailed instructions). Windows 7 users can also use System Repair Disc for booting; Windows 8 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 and 8 users can see the Restore a System Image article for instructions.
No backups? Use Puppy Linux for rescuing your personal files and reinstall Windows.