Child window close / Parent Session problem

[Xing Rong]

I have an asp page to authenticate users by checking their
uid and password:

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If (request("UID") = "user" and request("Password") = "pwd") then
session("UID") = "user"
Session.Timeout= 300
End if
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For other ASP pages, I have

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if not IsUser() then response.redirect "Login.asp"

-- Here is the function
function IsUser ()
IsUser = False
if session("UID") = "user " then IsUser = True
End function
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I also have a help link on each page. If you click the
link, a child window pops up using javascript.

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<a href="#" onClick="openWindow
('../Help/default.asp','','menubar=yes,scrollbars=yes,width=600,height=400')">
---------------------------------------

<script language="JavaScript" type="text/JavaScript">

function openWindow (theURL,winName,features) {
window.open(theURL,winName,features);
}

</script>
---------------------------------------

Now, My problem is: I click the link, a pop-up child
window opened, I close the child window, back to the
parent page and refresh, it redirect me to the login page. Looks like
the session is terminated.

However, If I first login to my Yahoo email account and re-
do the above, no problems at all.

Is it strange? Anyone has any ideas?

Thanks!