We have a Moodle site hosted in a Windows 2008-IIS7 server. It is a medium size site (number of users around 300, less than 60 courses) but it will be used to support another MSc.
In the 2.6, I have read that:
Windows OS is not suitable for large Moodle installations because PHP for Windows does not support integers above 2 millions - maximum file size is 2GB and database tables cannot contain more than 2 million records. Please consider 64bit Linux or other unix-like OS for all large Moodle servers.