When you say that the installation wizard finished, does this mean that you got through all 7 stages and the problem only showed up once you had a 'working' site and then tried to log in to The Admin area? Or did the problem show up at stage 3 of the installation wizard when you were first asked to choose and enter an admin username and password?
If the former, there may be a problem in your database.To rule it out, firstly, check that you are using the right Admin Username and Password.
Go to your online Database, select the table 'wa_settings' click browse, and look at the last two records (73 and 74) these have your admin username and password.
Secondly, check the paths for your store, still in table 'wa_settings', Check:
Record 1: (Root URL) - value should be: YourStoreDirectory/
Record 9: (Local Root) - value should be: /YourStoreDirectory/
Record 10: (Remote Root) - value should be: /YourStoreDirectory/
(The values will obviously reflect your own site and folders).
