VS2008 Setup conditional assignment of TARGETDIR

Question

I know this has probably been asked a million times but I don't see a clear answer. I've got a setup project under VS2008 where I want to check the registry to see if a particular app is installed and get its installation directory.

Ok fine, I do that with a Launch Conditions and assign it to my property: BlahInstallDir

Now in the Application Folder's DefaultLocation I have:

[BlahInstallDir]\Admin

for the assignment to TARGETDIR

That is ok if the Blah application is installed. If it is not, BlahInstallDir yields a null as one would expect. I would like to have a suggested DefaultLocation something like this in those cases:

[ProgramFiles][ProductName]\Admin

How in blazes can this blatantly simple conditional be performed (within VS2008)? Help and Thanks!