Yes I did infact modifie the registry manually a couple of days before.
I had gotten some advice to change the default installation directory.
Se the following thread for details: Standard installation path - Vista Help
I had changed the ProgramFilesDir and the ProgramFilesDir(x86) key.
It did not work, so after I had tried to install Google Earth I changed it back again. But then problems started.
I guess I should have taken a backup of the registry before I started, but unfortunately I did not do that.
I have tried to uninstall and install a program to see if that could help. But I still have the same problem
Pleas give me a hint on how to proceed.