Ok, for starters, this is getting into a "how do I configure Windows to send email via a relay-host"-type of question. This is, IIRC, a linux-form

Anyhow, what you should do:
- look at the help-pages provided by your ISP and locate the address of the relay-host (normally they provide an smtp-server that will accept emails from their subnet)
- figure out how to configure senmail (or whatever you use to send email on a windows machine) to send via a relay host