You can load the user up yourself

DotNetNuke.Entities.Users.UserInfo info = DotNetNuke.Entities.Users.UserController.GetUserByName(0, "Mike");
string password = DotNetNuke.Entities.Users.UserController.GetPassword(info, info.Membership.PasswordAnswer);

Now you have their password and can check it against the password that has been passed in, ensuring it is valid.  Is there a reason you want to manually validate the password instead of just calling the ValidateUser method?  Try using the ValidateUser overload as follows:

DotNetNuke.Security.Membership.UserLoginStatus logStat = DotNetNuke.Security.Membership.UserLoginStatus.LOGIN_FAILURE;
DotNetNuke.Entities.Users.UserInfo info = DotNetNuke.Entities.Users.UserController.ValidateUser(portalId, userName,
 password, string.Empty, string.Empty, "255.255.255.1", ref logStat);

and then check for logStat == DotNetNuke.Security.Membership.UserLoginStatus.LOGIN_SUCCESS and that info != null.

Thanks for all the help guys, It was actually pretty simple

If your usernames and passwords are stored as MD5 hashes in your database you cannot retrieve them. However, if you use encryption it can be reversed and gives you the password in plain text.

I used

string pass = Membership.Provider.GetPassword(ui.Username, null);

And then logged in the user. Quite easy