The Avast antivirus program can be run during bootup to scan for viruses outside of Windows. This feature is useful when a virus prevents you from booting into your computer, a virus prevents you from running antivirus software while it is active or if the virus changes files while they are being scanned. Avast has commands for many different functions so the program can be effectively run at the command prompt.
Restart your computer. Press "F8" to bring up your boot options menu.
Choose "Safe Mode with Command Prompt."
Type "aswcmd.exe" (without quotes), a space and a supported parameter to run commands. Common Avast command parameters are "/*" (without quotes) to scan local hard drives, "/c" (without quotes) to fully scan all files, "/h" (without quotes) to receive a help listing of all command prompt parameters and descriptions of the parameters, "/m" (without quotes) to test boot sectors and "/p" (without quotes) to have Avast automatically deal with viruses.
Avast Home does not offer any command prompt features, so you need one of the other editions of the program to access the commands.