unfortunately I'm not too familiar with windows 
If you want to give it a shot, you can download go compiler here + install instructions:
Once you have go in place you can use go get github.com/Azure/go-ntlmssp to get the dependency. Use git to clone beats repo into D:/Go/src/github.com/elastic/ and test your build by going to beat/heartbeat subdirectory and enter go build. If build works you can adapt task.go and recompile.
Feel free to open an enhancement request. There are a few libs supporting ntml (some seem to support v1 and v2), but evaluating those libs, adding support based on one lib (some libs seem to require quite some 'footwork') and building a fake http server for testing (that is I've no guarantee it will work with IIS, as I have no IIS available) will take quite some time. If you can confirm the 'Azure' one working (which is quite easy to use), we can add at least add experimental support to configure the authentication-method for the time being.
btw. I have no idea how to uninstall go development environment. Maybe you want to try building heartbeat in a virtual machine to throw away afterwards.