I have 2 questions about validation in ASP .NET

#1:
I have a custom validator that do not have any javascript code(validation
function), but i want to display the error in the javascript alert. in the
onLoad event of the Page. how can I do that?

#2:
I have a web form that has some validation controls. When I change this webform
everything is saved into a session variable.
so... when i go back to this form, my drop-downs state is restored ( with the
data in session) but the client validation ignore the state and can´t move.
ex:

when go back to the first webform:

this.AGENCIA_RETIRADA.Items.FindByValue(this.ObjReserva.AgRetirada.LocCode).Sele\
cted = true;

this works fine. But the validator said that the drop down named
"AGENCIA_RETIRADA" do not have a selected item.

can anyone help-me with this??