How to calculate Webserver Bandwidth usage
Solution 1:
You can use awstats, it's Open Source and can run on Windows
As this one and many others (webtrends...), it will depends on what you log in IIS. You will need to log the Sent & receive bytes. You should alos log the referer, user agents as good info.
If you didn't include these info, you won't get data for the past, so activate these as soon as possible