1. Adfender is an ad blocker and does not conflict with avast.
    Most of us use AdBlock Plus for this purpose.

  2. Not knowing what the pop-up said it is difficult to know.
    Did you click on the “more details” button in the pop-up?
    URL:Mal usually means a malicious URL. Generally avast blocks these infections.

  3. Did you run a “full system scan” before you did the reformat?
    If you did what was found or was anything at all found?

  4. Pondus knows what he’s talking about. Avast with MalwareBytes and MCShield make good partners.
    For a one time charge of $25 gets you a lifetime license to MBAM Pro. Purchase it before MBAM
    releases v2.0 or else it will be an annual charge.