Avast Anti Virus is a legit program.
But many security experts such as quietman7 are advising against its use, see this post: https://www.bleepingcomputer.com/forums/t/674207/should-i-replace-avast-with-windows-defender/?p=4471500
Suggest reading all of: Best Practices for Safe Computing - Prevention of Malware Infection
especially:
- Choosing an Anti-Virus Program
- Safe Steps for Replacing your Anti-virus - Why should you use Antivirus software?
- Supplementing your Anti-Virus Program with Anti-Malware Tools
- Choosing a Firewall
and then deciding which anti-virus program you want to use.