Why can I tether to Ubuntu for free but not Windows?
A lot of mobile network operators perform network analysis (deep packet inspection) of all transfered data. In that way they are able to identify which protocols are used. 

Identifiable protocols that indicate a desktop PC are for example:


Windows Update
Virus scanner update
Broadcasts sent by Windows


Another very easy and common way to identify the system is to read the UserAgent string the web-browser sends in each request. There are add-ons available for Firefox to fake the user-agent may you should try it and fake your browser to something unknown.