xp not accept address

[Michele Petrazzo - Unipex]

Hi list,
I have a working dhcp server that provide address to a network.
On some machine, normally with windows xp / vista, I see a strange
behavior when I want to change the address provided to that machine.
For explain better: I connect for the first time a winxp to the network
and it receive the address. After, I modify the dhcpd.conf removing that
address from range, reconnect winxp and it ask another time the same
address. Dhcp server say it that isn't available, but... win continue to
ask and stay with that address also if the server say that aren't valid!

Into the log I see:

DHCPREQUEST for /addrs/  from /mac/ via eth0:  unknown lease 
/unavailableaddrs/

What can I do? Is this a win bug or...?

dhpc3 server from debian lenny (3.1.1)

Thanks,
Michele