Win App Deployment include SQL 2008 Express - Check if any SQL Server version already exist

Hi there

I have a application that is deploying and installing correctly (Setup and Deployment Project in VS 2008) and has SQL 2008 Express as a pre-requisite. SQL Express installs fine if not found but I don't want it to be installed if any other version of SQL Server already exists on the target computer. What do I have to adjust in the Bootstrapper package (package.xml or product.xml)  for SQL Express to do this check? Is the change done within the Bootstrapper package? Do I have to create a Custom Action in my Deployment project for this check? Any help is appreciated.

Thanks

Dweezil