Lee, they are not running at the same time, they will be on seperate partitions on a dual boot system there is no conflict as there is only one AV in each OS partition.
@ rx74me
If you could post the error, perhaps it may give us a clue.
Have you scheduled a boot scan in XP that hasn't been run that might somehow be getting in the way of the dual boot.
Have you ever had this working?
Is this something that has just occured, r has it been like this since installing avast?
Have you installed/updated anything recently that could have had an effect?
Sorry I can't be more helpful, I don't use dual boot.