Calling JavaScript Function Of Boolean Type?

I am trying to call a Page Method using AJAX.

See the code

<asp:Button ID="btn_upload" runat="server" CssClass="btnEffects" Text="Upload"
onclick="btn_upload_Click" OnClientClick="return Validate();" />

Validate method is supposed to return true/false. The code for Validate method is

<script language="javascript">
function Validate()
{
var filename = $get('<%= txt_filename.ClientID %>').value;
PageMethods.IsValidFile(filename,OnSuccess, OnFailure);
// IsValidFile is a Page Method of bool return type
}
function OnSuccess(result)
{
if ( !result)
{
alert('File '+ $get('<%= txt_filename.ClientID %>').value + ' does not exist');
return false;
}
else
{
return true;
}
}
function OnFailure(error)
{
}
</script>

My problem is that even after showing the alert 'File somefilename does not exist'. The whole page refreshes (i.e, postback happens)