If you search the board you’ll find that this is not anything to worry that much.
It’s the ‘famous’ avast error message http://forum.avast.com/index.php?topic=4818.msg65672#msg65672
42100 AVAST_POLYVIR_EXCEPTION Exception in polymorphic viruses code

I never found a good ‘solution’ for it…
http://forum.avast.com/index.php?topic=7385.msg65808#msg65808

It was never good explained anywhere…
http://forum.avast.com/index.php?topic=18263.0
http://forum.avast.com/index.php?topic=15302.0
http://forum.avast.com/index.php?topic=23382.0