Auto install W2K8 Server and SQL Server on new VM

Hi Guys

I frequently need to build new W2K8/SQL Server2008R2 VM's. These must be built from scratch (as part of our QA routine - i.e. they cannot be a previously installed VM that's copied and renamed). The only thing that really changes from build to build is the hostname and SQL instance name. I can automate installation of SQL, but not sure how I'd go about building a new VM from scratch (using VMware 9 currently) and an ISO of the installer. I know you can create some kind of answer file, but I've never used one

Can anyone shed any light on how to do this?

Thanks

Mark