I have managed to search for a free trial of windows 7, and found them at Microsoft MSDN and TechNet, however you have to be a subscriber in order to have the trial key which I think the trial would last between 90 to 120 days. If you asked me few weeks ago you could have it by now, but Microsoft has officially closed the release candidate subscription.
Read this article
and this one
for free RTM license explanation for subscribers.