I first got this error message when trying to install:
The upgrade patch cannot be installed by the Windows Installer service because the program to be upgraded may be missing, or the upgrade patch may update a different version of the program. Verify that the program to be upgraded exists on your computer and that you have the correct upgrade patch.
Then I remembered that I installed the beta of SP1 so I had to uninstall it first. Make sure to select Show updates in Add or Remove Programs. I forgot that first and could not find it.
The install really takes looong time as you are informed about when the install starts.