Login to system from service?

I have a windows service and want to automatically log a user into windows from it - I have seen this done with logmein etc...

I have looked into GINA and CredentialProviders, but haven't really found an article on how to do this; most forums just say "look into GINA and CredentialProviders"

I know the username I want to log in and the user will ALWAYS have no password.

Any help would be appreciated.

Thanks,
Kris