Can't switch to a different user

I have set up 3 user accounts on my laptop with Vista Home Premium. After
starting the computer I can log into any one of these accounts but I can't
Switch User - I get all three user accounts on the screen and on choosing
one of the remaining two accounts I get "The System Event Notification
Service service failed the logon. Access Denied".

I can only go back to the account I initially logged into.

Secondly, if I log into an account and then Log Off, I can't log into any of
the three accounts - again I get the same message as above. I now have to
reboot the machine to be able to log on.

Anyone have a solution to this?

Jan