Hi,
I have recently built my self a computer, with windows 8 and I was wondering if my PC is secure enough with windows 8 and avast (free version). Thank you
Depends on what you do.
If you're careful about what sites you visit and don't download 3rd party applications then you should be okay with the integrated security.
I'd advise that you remove Avast, and keep Windows Defender as your primary antivirus.