table style="text-align:center"
doesnt make the table center aligned.
You need to wrap a container div around your table and make that center aligned.
Use like this :
<div style="width:100%; text-align:center">
<table style="text-align: center" width="500" border="0"
cellpadding="0" cellspacing="0">
<tr>
<td align="center">
<asp:Label ID="Label1" runat="server" SkinID="TableHeading"
Text="Report" Width="100%"></asp:Label></td>
</tr>
<tr>
<td align="center">
<asp:Label ID="Label2" runat="server" Text="Access No">
</asp:Label>
<asp:TextBox ID="txtaccno" runat="server"></asp:TextBox>
</td>
</tr>
</table>
</div>
I hope this would work.