I was using the login control in Visual Studio 2005 the other day, and was surprised to note that the default provider requires a password of at least 7 characters at least 1 of which must be non-alphanumeric (e.g. @ or #).
That is a tough regime for a simple website - how do you change it? Have a look at this article here. Once done, you can still use the web site configuration wizard and it will adhere to the complexity defined in your web.config in preference to the default one in machine.config.