gooders5,
Was the virus detected after the update was complete, or did the error message occur during download?
It is highly unlikely that the Avast update contained a virus.
Try running a full virus scan without the update.
Set Avast to scan archives and set it to Thorough scan.
If virus is detected, clean it or move it to the Chest, then delete. If no virus is detected, try to complete the update again, then rerun scan.
The Dark Angel virus affects com files, and remains in memory. It is an exe file so do not click on it in any other place but the CHEST.
Set Avast to run a check of the memory at startup and reboot. If Avast finds it, you should be able to clean it.
If Avast is telling you it is there, more than likely, it is.
It more than likely is a false positive, but you need to run the tests I recommended first.
Note: I have read that Avast can detect it's own updates as a virus. I think the message board had something on this. I don't think you have anything to worry about, but run the tests to be sure.