If you install Windows 10, you are protected against hackers from the git-go by Windows Defender. You can then choose to keep Defender running, or you can download and install another anti-virus application like Avast, Avira, or Microsoft’s own Microsoft Security Essentials (I’m running that one as we speak).
From the Microsoft website: Windows 10 is safer and more secure thanks to Windows Defender and Windows Firewall.
When you start up Windows 10 for the first time, Windows Defender is on and working to protect your PC by scanning for malicious software. It will turn itself off if you install another antivirus app.