Thanks for your reply - I have found the reason for my problem. I kept copying the plugins in the "plugins" folder within my installation. For some reason things seem to be different in Vista though.
You have to go to C:\Users\YOURNAME\AppData\Roaming\MAXON\CINEMA 4D R11_79C2D4E5 and create a directory called "plugins". This where you have to copy the plugs and everything is fine ...
