1. Download a ISO of the version of Windows you want to use. IIRC almost all copies of Windows since Vista, at least the corporate and unattended install versions allow you to use it without a valid key and won't start bugging you about activating until 30 days after install.
2. When the 30 days is up either purchase a legit key and activate or download a activation crack. The Windows 7 activators are flawless, no issues with updates, malware or any other undesirables. If you go the cracked route, donate what you would have spent on Windows to charity, email Microsoft and tell them why you decided to use Windows but not pay for it and what would have to change to make you use AND pay for a future version.
BTW, the "Click here to login and comment blah blah" link at the bottom of forum posts no worky