When installing Snort on Windows […], the requirements include Winpcap and Barnyard.
Unfortunately Barnyard for Windows is not that easy to find: as you can read here, you can find a packaged Barnyard for Windows at http://codecraftconsultants.com/Barnyard/, including source.
It may be useful, although not ready for production deployement.
Other interesting Snort over Windows resources:
Sans.org -Securing a Windows Snort Sensor for Hostile Environments
Snort Installation Manual Snort, MySQL, ACID, & IIS
WinSnortIIS local copy