The Invalid User error message can only be caused by any one or combinations of the following..

1 - Using an invalid / incorrect username
2 - Using an invalid / incorrect subscription id
3 - You have left a space at the end or beginning of your Username or Subscription ID.
4 - Your account hasn't yet been approved
5 - Your feed has been suspended

Your username is the name you choose on registration.
Your subscription id can be found on the first page of your iCodes login area.

If you 'Generate new Subscription ID' this will need to be reflected in your website script.

You can test your username and subscription ID by copy/pasting a generated webservices request into your browser address bar, this will output the XML feed in your browser if the details are correct.