Mike,

That was the error using the Wiondows login. With the Standard login it comes up with 

Login Failed, remember that Passwords are case sensitive

(as mentioned by thond in the original post).

Hi Mike & Dougie:

In my case this is a fresh install of DNN5.
DNN version: 05.00.00
AD Provider version: 5.0.1
Server/IIS version: Windows xp professional SP3/IIS 5.1

As metioned by Dougie, this error will happen only when I use Windows login to login with a existing AD account(domainName\thond) at first time to DNN5 Application and Admin/Event Viewer describe that :

1/22/2009 3:54:16 PM   Page Load Exception       AssemblyVersion: 05.00.00; PortalID: 0; PortalNa ...
AssemblyVersion: 05.00.00
PortalID: 0
PortalName: test
UserID: -1
UserName: 
ActiveTabID: 38
ActiveTabName: Home
RawURL: /dotnetnuke5/Home/tabid/38/ctl/Login/Default.aspx?returnurl=http%3a%2f%2ftwn-hcu-thond%2fdotnetnuke5%2fHome%2ftabid%2f38%2fDefault.aspx
AbsoluteURL: /DotNetNuke5/Default.aspx
AbsoluteURLReferrer: http://twn-hcu-thond/dotnetnuke5/Home/tabid/38/ctl/Login/Default.aspx?returnurl=http%3a%2f%2fxxx-xxx-thond%2fdotnetnuke5%2fHome%2ftabid%2f38%2fDefault.aspx
UserAgent: Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1; .NET CLR 2.0.50727; .NET CLR 3.0.04506.648; .NET CLR 3.5.21022; .NET CLR 3.0.4506.2152; .NET CLR 3.5.30729; FDM; MAXTHON 2.0)
DefaultDataProvider: DotNetNuke.Data.SqlDataProvider, DotNetNuke.SqlDataProvider
ExceptionGUID: d075fc47-9779-402f-9e19-e2e1e269ae37
InnerException: Unknown UserCreateStatus value encountered Parameter name: UserRegistrationStatus
FileName: 
FileLineNumber: 0
FileColumnNumber: 0
Method: DotNetNuke.Entities.Users.UserController.GetUserCreateStatus
StackTrace: 
Message: DotNetNuke.Services.Exceptions.PageLoadException: Unknown UserCreateStatus value encountered Parameter name: UserRegistrationStatus ---> System.ArgumentException: Unknown UserCreateStatus value encountered Parameter name: UserRegistrationStatus at DotNetNuke.Entities.Users.UserController.GetUserCreateStatus(UserCreateStatus UserRegistrationStatus) at DotNetNuke.Authentication.ActiveDirectory.Login.WindowsAuthorization(UserLoginStatus& loginStatus, String IP) at DotNetNuke.Authentication.ActiveDirectory.Login.cmdLogin_Click(Object sender, EventArgs e) at System.Web.UI.WebControls.Button.OnClick(EventArgs e) at System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument) at System.Web.UI.WebControls.Button.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument) at System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument) at System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData) at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) --- End of inner exception stack trace ---
Source: 
Server Name: XXX-XXX-THOND

Although the error happened, i go to Admin/User Accounts to find that the "domainName\thond" AD accounthad been added. Then login again will success.

It seems like the situation in the thread:
Error when loggin in with an AD User not in DNN database http://www.dotnetnuke.com/Community/Forums/tabid/795/forumid/89/threadid/211476/scope/posts/Default.aspx