Either this Page.ClientScript.RegisterStartupScript(Me.GetType(), "Myscript", "javascript:alert('Your record deleted')", True) has to go in your Default.aspx page and execute based on certain flag because the server forces client browser to redirect and thus all scripts registered with the current page will not run.
OR:
Keep your script where it is, but instead of having only alert calll a function:
function deleted()
{
alert('Your record deleted')
window.location = "Default.aspx"
}
and get rid of the server-side redirect.
in this case your script will execute on the current page, display the message and redirect.