Please excuse the mangled terminology ;)
You have a Windows DNS server handling internal traffic for the yourco.com domain.
You want just the external website (www.yourco.com) to be ultimately resolved from a public DNS server such as your ISP's (e.g. should you ever change the IP address of your website you wouldn't have to update both your companies private DNS server and the public DNS records)
How can you achieve this using Windows DNS server?
If I understand the question correctly, what you want is to make your DNS server (WDNS) primary for the zone.
It means that your ISP's DNS should set your WDNS set as primary server for the zone yourco.com and ISP's own DNS server should accept yourco.com zone transfers from it and act as secondary(slave) DNS server.