Here is a possible solution
0. Grant temporarily "full access" to the RSA directory (C:\Program
1. temporarily rename the MachineKeys directory to MachineKeysOld
2. create a new directory MachineKeys
3. run the tv/guide setup in Media Center
4. if step 3 was succesful, copy the newly generated key from the new
MachineKeys directory to the MachineKeysOld directory. It may happen that the
very same key already exists and can't be replaced, even by an administrator.
In that case manual appropriation of the file is required.
5. Remove the new directory MachineKeys
6. Rename MachineKeysOld back to MachineKeys.
7. Restore the initial permissions of "Everyone" for the RSA directory.