I can connect to openvpn server under win7/vista successfully

I have used lots of time struggling on this issue...
Luckily, I got successful finally
If you want to use push "redirect-gateway" function, you have to type the following commands

step1:  iptables -t nat -A POSTROUTING -o eth0 (change to vpn server's interface which hold the real IP)  -j SNAT --to 63.127.146.199 (changed to vpn server's real IP)
step2: iptables -A FORWARD  -j ACCEPT
step3: iptables -L -t nat     (list all rules,have a look of result)
step4: echo 1 > /proc/sys/net/ipv4/ip_forward

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Edit your server.conf with adding the following lines
push "dhcp-option DNS xxx.xxx.xxx.xxx" where xxx.xxx.xxx.xxx is your ISP's DNS
push "redirect-gateway"
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Edit client.ovpn under win7/vista with the adding the following lines for successful connection
route-method exe
route-delay 10

Finally, restart your openvpn server. Everything is supposed to be ok then!  Enjoy it:)
well...your solution is quite good...
How to use PPTP server ?
ya...the setting is really easy and PPTP is good to use, thanks for your recommendation!