This path is stored in the registry under a string value named InstallLocation. The actual registry path varies depending on the installer.

MSI / Windows Installer Project:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall

InstallShield and Wise:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\App Paths

References
http://social.msdn.microsoft.com/Forums/en/winformssetup/thread/fe4e31b9-2eee-45db-a30e-119f87d9df4d