How To Detect If SQL Server 2005 Is Installed?

I need to be able to programmatically detect whether or not SQL Server 2005, anything from Express on up to the full package with all the bells and whistles, is installed on the computer regardless of which service pack (if any) is installed. I found a forum article that provides a Registry setting that I could look at, but in the reply to the original post, it recommends using WMI instead of Registry keys. Also, I don't know whether or not the Registry key given for SQL Server 2005 would be present for even the minimal package. (Our requirements are for at least the Express package, and I understand that there's an even more basic package than that.) I'm maintaining a couple of installers that use 5-year old InstallShield technology (and Visual Studio 6 for any C/C++ code that I have to create) that have to check whether or not SQL Server 2005 Express (or a fuller package) is installed. I have done no SQL Server programming, so I've never even heard of WMI. Also, because these are installers, they have to work out of the box with no requirements to install anything beforehand to allow the installer to work. (We are working on an MSI-based installer with a bootstrap executable (the latter built using Visual Studio 2005) that will replace the older installers in the next version of our software, but we still need to support the older installers for now. Therefore, an out of the box solution that works with the older technology that we are still using for the older software is imperative. As the older software supports Win2k, the SQL Server detection solution must do so, too.)