Boot time scan only works if you can boot into Windows. Most malware now a days disables Safe Mode. Especially randsomware. Hence the need for a rescue disk. The following are Free:

  1. Kaspersky
  2. Comodo
  3. AVG
  4. Dr. Web
  5. Bit Defender
  6. Avira

So now lets have an Avast Free bootable rescue disk ;D