All right I got it!
I just changed.
RegistryKey regKey = Registry.LocalMachine.OpenSubKey(@"SOFTWARE\Microsoft\VisualStudio\9.0\Setup\VS");
vsInstallationPath = regKey.GetValue("ProductDir").ToString();
to:
RegistryKey regKey = Registry.LocalMachine.OpenSubKey(@"SOFTWARE\Microsoft\VisualStudio\6.0");
vsInstallationPath = regKey.GetValue("InstallDir").ToString();
Thanks a lot for your help!