protected void txtAsset_TextChanged(object sender, EventArgs e)
{
  // Shouldn't this IF be ( txtAsset.Text = "SELECT assettag 
  // FROM computers" from DB).
  //
  if (txtAsset.Text.StartsWith("a", StringComparison.InvariantCultureIgnoreCase))
  {
// Shouldn't this part be : txtSerial.Text = SELECT serial 
//  FROM computers WHERE assettag = "xxxxxx" ?
//
    txtSerial.Text = "Found";
// Do I need to post it back to the viewstate? In truth I want the form to reply with a lot more fields (15 or so), I only mentioned 3 in the post for simplicity. In your book you mention not to go overboard with the viewstate cause of bandwidth issues.

    ViewState["ItemId"] = Guid.NewGuid();
  }
  else
  {
    txtSerial.Text = "NOT Found";
    ViewState["ItemId"] = null;
  }
}

myCommand.Parameters.AddWithValue("@pcAssetTag", txtPCAsset.Text);

string sql = "SELECT pcAssets.pcAssetTag, pcAssets.pcSerialNumber, pcModels.pcModelName FROM pcAssets INNER JOIN pcModels ON pcAssets.pcModelID = pcModels.pcModelID WHERE pcAssetTag = @pcAssetTag";

<tr>
  <td>PC Description</td>
  <td>
    <asp:TextBox ID="txtPCDescription" runat="server"></asp:TextBox>
    <asp:DropDownList ID="ddlPCDescription" runat="server" 
      DataSourceID="SqlDataSource1" DataTextField="pcModelName" 
      DataValueField="pcModelName"></asp:DropDownList>
    <asp:SqlDataSource ID="SqlDataSource1" runat="server" 
      ConnectionString="<%$ ConnectionStrings:PC Assets DatabaseConnectionString1 %>" 
      SelectCommand="SELECT [pcModelName] FROM [pcModels]">
    </asp:SqlDataSource>
  </td>
</tr>

ddlPCModel.SelectedIndex = ddlPCModel.Items.IndexOf
       (ddlPCModel.Items.FindByText(myReader.GetString(2)));