Routing VS Proxy problem

[Gentoo Server]

iptables -P FORWARD ACCEPT
echo 1 > /proc/sys/net/ipv4/ip_forward
iptables --table nat --append POSTROUTING --out-interface ppp0 -j MASQUERADE
iptables --append FORWARD --in-interface eth0 -j ACCEPT
iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 80 -j REDIRECT --to-port 3128


when I try to windows update my xp client I get no connection
when I set proxy 3128 everything is ok

how can I nat/masq that I can use windows update without proxy using?

Thnx