I had a similar problem.
It seems that every time an avast update is downloaded the virtual HD image of the suspended parallels desktop VM is detected by avast to contain "Dead-B", "Win32:Small-HUF[Trj]", and a couple other things that I don't remember and has not taken note of.
Having not found a way to contact support, and since my disk image are just apps and has no sensitive data in them, I opted to send the file to the "avast Lab" to report a "false positive" detection by avast.
Not getting any feedback from avast, I played around the UI and found the I can exempt the virtual PC folder from the full system scan.
Having read in Bas' post that it is the same solution that the avast support suggests, I feel more than a little bit secure that what I did -- exempting the Parallels Desktop folder from the scan -- was the right thing to do.
Thanks!
WISHLIST: for avast team to fix this problem.