openvpn does not support NAT at all... this is a feature of your OS not openvpn.
Make it easy, change one of the subnets
but for the record, windows does NAT
http://www.nanodocumet.com/?p=14 for windows XP
http://www.windowsnetworking.com/articles_tutorials/NAT_Windows_2003_Setup_Configuration.html for server 2003