Here is a link to a free version of Windows 7 Enterprise x64:

http://technet.microsoft.com/en-us/evalcenter/cc442495

Free for up to 6 months, then you reinstall for another 6 months. All legal.



EDIT:
Looks like it can be rearmed for longer than 700+ days
http://www.oqotalk.com/index.php?topic=4767.0