On your desktop will be a file called MBR.dat could you add that to the virus chest and then upload to the virus lab - for comments put in undetected MBR infection

To add to the virus chest :

Open Avast and select Maintenance > Virus chest
Right click in the white area to the right and select Add
Browse to MBR.dat and select
Once it is in the chest right click the file and select Send to Virus labs

Is Avast still alerting ?