Your sme server has to be set gateway mode for portforwarding to work
You could create a hostname on the sme box
say ---intranet in your domain name list you wish to use then
set it for forward or go to the ip address of your intranet server
example in sme hostname screen
www.domain.com-----selfftp.domain.com-----self
mail.domain.com----self
intranet.domain.com----ip address of the windows box
NOTE:
If you want to call this from the (internet) you will need a hostname for intranet.domain.com added at your DNS service
I use zoneedit so you are on your own from here
SME SERVER MUST BE IN GATEWAY MODE FOR THE FORWARDING TO WORK