I came across this scenario the other day. I had to install PostgreSQL 9.0 on a windows box that already had an old PostgreSQL install and hence a local postgres account already setup. Using the same postgres account was not an option as a) I didn’t know the password and b) if the the password ever changed nobody would update the PostgreSQL 9.0 service entry with the updated password.
Fortunately there is an easy way to do this. Launch the one-click installer from the command line with the following arguments
postgresql-9.0.01-1-windows.exe --serviceaccount username
This will allow you to complete the installation as usual but the service account username will be the value passed. You can confirm this on the screen which asks you for the password. You can also complete the whole installation unattended just Launch the one-click installer with –help argument for a full list of options.