I have added a user to Windows Share Point Services and given him
administrative right, this user is not able to add items to a few of
the list like the discussion list, the event lists.. he is able to
add document to a document library but not to any other lists.

When the user tries to add items to a list, the following error is
displayed

"The security validation for this page is invalid. Click Back in
your Web browser, refresh the page, and try your operation again."

any suggetstions?

we are clean bowled here trying ...