It's true that you can extend the Window's license in any version for 120 days. The trick is to do this right at the end of thirty days from the first day you install Windows 7.
All you have to do is invoke the slmgr -rearm command and your done. But this command only works 3 times and then your forced to buy the license or install another operating system.
Now at least you have time to evaluate the software for a total of 120 days.
