How to do this validation in asp.net?

I am new on asp.net programming.
Here I got a problem about validator
I want to write a login.aspx page, for example, there is UserName box. I need to validate two things on UserName box,
1. It must be filled. If it is empty, I will show a red message right after this box, like "UserName is empty!"
2. It must be a valid user in our database. So on server side, we will check this UserName in our database. If it is not in our database, we will show something like "Invalid user name..."
I know it is very easy to do the 1st rule with the RequireFieldValidator. But in order to do the rule 2, I need to put a Lable at the same place where the validator is. Right? But the two won't stay at the same place. So I tried to use a CustomValidator, and write the ClientValidationFunction like this:
<script language="javascript">
function ValidUserName(oSrc, args) //For rule 1
{
if(args.Value.length > 0)
args.IsValid = true;
else
args.IsValid = false;
}
</script>
And also, I have a server validate function which will process the rule 2.
But the problem is the client validate function never get called. So , do you guys have a better solution for this? I really appreciate.