Activation error

I am getting an error that says: The Windows Vista Home Premium product key
you typed is invalid for activation. I read the post someone else put up
about the same problem & tried following the suggestions (typing net start
slsvc in the cmd.exe application) and I get the following: "The requested
service has already been started. For more help..." I typed in what it said
for more help & got the same message. What else can I do?