Textboxes validation

I have a form which allows user to insert data to SQL 2005. I am having trouble performing exact validation on 2 of the boxes. First box is txtComputerName which is a primary key in TableA and second textbox is txtUserName which is a foreign key in TableA and primary key in TableB.
protected void txtUserID_TextChanged(object sender, EventArgs e)
{
this.txtUserName.Text = this.txtUserName.Text.ToUpper();
}
protected void c ...