DHCP

We have DHCP setup on our windows 2000 servers and our
2000, XP and magority of win98 clients do not have a
problem obtaining and renewing IP addresses but with some
we get the following problem

We have quite a few Win95 & 98 clients running DHCP but
when the lease is due to expire they don't seem to release
and renew the address properly. If you run a winipcfg it
has a valid IP address put you cannot ping anything and
you sometimes get an IP conflict between the older
versions of windows. Apart from doing a manual release
and renewal are there any settings or patches that we can
use to correct this problem?

Thanks