While not exactly what you asked, this thread should give you enough to run with...http://community.installshield.com/showthread.php?t=165331&highlight=INSTALLDIRHope its helpful
SYSINFO is an InstallScript variable. Conditions should reference Windows Installer properties. Try using VersionNT (Vista = 600), WindowsBuild (Vista = greater than 5600). More help can be found in the Windows Installer help under Windows Installer ...
Read up on the Windows Installer properties REINSTALL and REINSTALLMODE, which are used during a repair/reinstall.It all revolves around the key identified for the component(s) that make up the feature(s), and under what conditions it will reinstall ...