DHCP Client With Reservations Not Renewing

My Windows 2003 clients with DHCP reservations are not requesting renewals
of their reserved IP when the lease duration expires. This causes problems
because the DHCP server is faithfully deregistering the DNS forward and
reverse lookups for these machines at expiration of the lease.

How can force the DHCP client to at minimum reactivate its reservation at
the end of the lease period? Even better, how could I force a renewal
every N days, even when the lease is already active?

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Will