It depends on a) what version of avast it is, 4.8 with self protection and )b what it is that you are likely to be doing, edit avast4.ini, etc.
So if it is 4.8 and you are likely to be rummaging around in avast you 'might' need to disable self-defence.
You don't say what the Vista problem is, if it is related to avast in any way so we can't be very specific.