Avast antivirus comes with several default scans.

• Quick scan - Scan of your computer's system volume (usually C:\ drive). Only files with extensions such as "exe", "com", "bat" etc. are scanned. Only the beginning and at the end of the file, where viruses are normally found, are tested.
• Full System Scan - A more detailed scan of all your computer's hard disks. All files are scanned according to their content. The whole file is tested.
• Removable Media Scan - Scan any removable media (e.g. USB flash drives, external hard drives) when attached to your computer.
• Select folder to scan - Scan only a specific folder or multiple folders.
• Boot-time scan - Perform a scan at bootup to detect and remove viruses which cannot be detected after start up. Click on Schedule Now to automatically scan the next time your system is restarted.

On the Scan screen select a pre-defined scan from drop-down menu and click Start. After scanning is complete you can view your results and take action on any threats found.

From Scan ▸ Smart Scan you can also run Smart scan, which is a predefined virus scan combined with other Avast scanning tools.

You can manage scan settings by clicking Settings... below the scan name. However it is not recommended to change the default scans but instead to Create a custom scan at the bottom of the screen.