Hi Elle1971,
Think the site is clean, if there is a block it is a general IP block, see what Pondus gave us to ponder on.
So report the false positive to Avast at:
http://www.avast.com/contact-form.phand see whether they will unblock your domain from that general IP block,
but again that is up to the avast team members responsible for blocking, e.g. Milos et all.
We are into this scanning and evaluation "just for the good of our souls"
and to improve on the security awareness of users and website owners alike,
Damian aka polonus