Parallel installation of 5.1. and 5.0.1 on windows?

I'm new to Pentaho, but I have 5.0.1 working on windows. I want to get 5.1 working on a different port to configure, test and migrate to it.
Changing the port is fine. But when I run tomcat2.exe to start the server, the Java parameters are all coming from the existing 5.0.1 installation (such as the classpath, which points to the 5.0.1 installation).

Is it possible to run two instances in parallel, and if so, what I am doing wrong? What path is being searched to find the java runtime settings for Pentaho server?

thanks
Tim