Windows EC2 initial config from AMI after autoscale boot up

I'm looking at autoscaling an off-the-shelf Amazon provided Windows AMI (as opposed to a custom one).

After a new instance gets booted (and before!! it gets added to load balancer), I want some initialization tasks injected and ran on the new instance (say, running a powershell script).

Secondly, the loadbalancer needs to know to only start sending requests to the new EC2 instance after this startup script has finished (not sure how to do this either).