Well, Microsoft did something completely insane - they changed a prototype of a system exported function in Vista RC1. I guess it doesn't sound that bad to most of you, but it's really crazy - you are not expected to do that, ever, and certainly not in RC1 build!
It's like a pharmaceutical company changing the formula for their medicament, without changing its name, or actually telling anybody about it. So, everybody using the good old dosage, as they are used to, suddenly dies from overdose
So, even though the blocking of avast! driver was probably done by mistake or bug in Vista, the latest official build of avast! wouldn't work anyway in the RC1 build of Vista (it causes bluescreen, as some found a few posts above). I think Vlk will release an updated version when he comes back from holiday.