how to call customvalidator on text change event

I have one asp.net page with code behind vb.Im using  asp.net 1.
My requirement is  to validate the existance of a number in database on change event of textbox.
 For that Im using a customvalidator,
So that on TextChanged event of the textbox a
server side validation is to be done to check whether
the entered number exists in the database.Ive given autopostback=true.
         But I cudn't call the custom validator on text change event.
 Now the validation is executing correctly on button click only.
But after displaying the error message the data gets submitted to the database.
How can i prevent that and also can I call the custom validation on text change event?

My code is as below:
<table>
<TR>
<TD><asp:label id="lblNo" runat="server" > No:</asp:label></TD>
<TD><asp:textbox id=&qu ...